Essays on Software Engineering
Frederick P. Brooks, Jr,
ABOUT THE AUTHOR r F 6
Frederick P. Brooks, Jr., is Professor and Chairman of the Com- ^ i (
puter Science Department at the University of North Carolina at
Chapel Hill. He is best known as the ''father of the IBM Sys-
tem/360," having served as project manager for its development
and later as manager of the Operating System/360 software
project during its design phase. Earlier, he was an architect of the
IBM Stretch and Harvest computers.
At Chapel Hill, Dr. Brooks has participated in the establishment
and guiding of the Triangle Universities Computation Center and
the North Carolina Educational Computing Service. He has pub-
lished Automatic Data Processing, the System/ 360 Edition of Auto-
matic Data Processing, and chapters in several other books.
The Mythical Man-Month
Essays on Software Engineering
Frederick P. Brooks, Jr.
University of North Carolina, Chapel Hill
ADDISON-WESLEY PUBLISHING COMPANY
Reading, Massachusetts • Menlo Park, California
London • Amsterdam • Don Mills, Ontario • Sydney
Cover drawing: C. R. Knight, Mural of La Brea Tar Pits. Courtesy of the
Photography Section of the Natural History Museum of Los Angeles
Copyright © 1975 by Addison-Wesley Publishing Company, Inc. Philippines
copyright 1975 by Addison-Wesley Publishing Company, Inc. Copyright © 1972
by Frederick P. Brooks, Jr.
All rights reserved. No part of this publication may be reproduced, stored in a
retrieval system, or transmitted, in any form or by any means, electronic, mechan-
ical, photocopying, recording, or otherwise, without the prior written permission
of the publisher. Printed in the United States of America. Published simultaneously
in Canada. Library of Congress Catalog Card No. 74-4714.
To two who especially enriched my IBM years:
Thomas J. Watson, Jr.,
whose deep concern for people still permeates his company,
Bob O. Evans,
whose bold leadership turned work into adventure.
Digitized by the Internet Archive
in 2013 with funding from
In many ways, managing a large computer programming project is
like managing any other large undertaking — in more ways than
most programmers believe. But in many other ways it is different
— in more ways than most professional managers expect.
The lore of the field is accumulating. There have been several
conferences, sessions at AFIPS conferences, some books, and pa-
pers. But it is by no means yet in shape for any systematic text-
book treatment. It seems appropriate, however, to offer this little
book, reflecting essentially a personal view.
Although I originally grew up in the programming side of
computer science, I was involved chiefly in hardware architecture
during the years (1956-1963) that the autonomous control pro-
gram and the high-level language compiler were developed. When
in 1964 I became manager of Operating System/360, I found a
programming world quite changed by the progress of the previous
Managing OS/360 development was a very educational expe-
rience, albeit a very frustrating one. The team, including F. M.
Trapnell who succeeded me as manager, has much to be proud of.
The system contains many excellencies in design and execution,
and it has been successful in achieving widespread use. Certain
ideas, most noticeably device-independent input-output and ex-
ternal library management, were technical innovations now
widely copied. It is now quite reliable, reasonably efficient, and
The efl^ort cannot be called wholly successful, however. Any
OS/360 user is quickly aware of how much better it should be.
The flaws in design and execution pervade especially the control
program, as distinguished from the language compilers. Most of
these flaws date from the 1964-65 design period and hence must
be laid to my charge. Furthermore, the product was late, it took
more memory than planned, the costs were several times the esti-
mate, and it did not perform very well until several releases after
After leaving IBM in 1965 to come to Chapel Hill as originally
agreed when I took over OS/360, I began to analyze the OS/360
experience to see what management and technical lessons were to
be learned. In particular, I wanted to explain the quite different
management experiences encountered in System/360 hardware
development and OS/360 software development. This book is a
belated answer to Tom Watson's probing questions as to why
programming is hard to manage.
In this quest I have profited from long conversations with R.
P. Case, assistant manager 1964-65, and F. M. Trapnell, manager
1965-68. I have compared conclusions with other managers of
jumbo programming projects, including F. J. Corbato of M.I.T.,
John Harr and V. Vyssotsky of Bell Telephone Laboratories,
Charles Portman of International Computers Limited, A. P. Ershov
of the Computation Laboratory of the Siberian Division, U.S.S.R.
Academy of Sciences, and A. M. Pietrasanta of IBM
My own conclusions are embodied in the essays that follow,
which are intended for professional programmers, professional
managers, and especially professional managers of programmers.
Although written as separable essays, there is a central argu-
ment contained especially in Chapters 2-7. Briefly, I believe that
large programming projects suffer management problems different
in kind from small ones, due to division of labor. I believe the
critical need to be the preservation of the conceptual integrity of
the product itself. These chapters explore both the difficulties of
achieving this unity and methods for doing so. The later chapters
explore other aspects of software engineering management.
The literature in this field is not abundant, but it is widely
scattered. Hence I have tried to give references that will both
illuminate particular points and guide the interested reader to
other useful works. Many friends have read the manuscript and
some have prepared extensive helpful comments; where these
seemed valuable but did not fit the flow of the text, I have included
them in the notes.
Because this is a book of essays and not a text, all the refer-
ences and notes have been banished to the end of the volume, and
the reader is urged to ignore them on his first reading.
I am deeply indebted to Miss Sara Elizabeth Moore, Mr. David
Wagner, and Mrs. Rebecca Burris for their help in preparing the
manuscript, and to Professor Joseph C. Sloane for advice on illus-
Chapel Hill N.C F. P. B., Jr.
Chapter 1 The Tar Pit 3
Chapter 2 The Mythical Man-Month 13
Chapter 3 The Surgical Team 29
Chapter 4 Aristocracy, Democracy, and System Design 41
Chapter 5 The Second-System Effect 53
Chapter 6 Passing the Word 61
Chapter 7 Why Did the Tower of Babel Fail? 73
Chapter 8 Calling the Shot 87
Chapter 9 Ten Pounds in a Five-Pound Sack 97
Chapter 10 The Documentary Hypothesis 107
Chapter 11 Plan to Throw One Away 115
Chapter 12 Sharp Tools 127
Chapter 13 The Whole and the Parts 141
Chapter 14 Hatching a Catastrophe 153
Chapter 15 The Other Face 163
Notes and references 179
Een schip op het strand is een baken in zee.
[A ship on the beach is a lighthouse to the sea. ]
C. R. Knight, Mural of La Brea Tar Pits
Photography Section, Natural History Museum of Los Angeles County
The Tar Pit
No scene from prehistory is quite so vivid as that of the mortal
struggles of great beasts in the tar pits. In the mind's eye one sees
dinosaurs, mammoths, and sabertoothed tigers struggling against
the grip of the tar. The fiercer the struggle, the more entangling the
tar, and no beast is so strong or so skillful but that he ultimately
Large-system programming has over the past decade been
such a tar pit, and many great and powerful beasts have thrashed
violently in it. Most have emerged with running systems — few
have met goals, schedules, and budgets. Large and small, massive
or wiry, team after team has become entangled in the tar. No one
thing seems to cause the difficulty — any particular paw can be
pulled away. But the accumulation of simultaneous and interact-
ing factors brings slower and slower motion. Everyone seems to
have been surprised by the stickiness of the problem, and it is hard
to discern the nature of it. But we must try to understand it if we
are to solve it.
Therefore let us begin by identifying the craft of system pro-
gramming and the joys and woes inherent in it.
The Programming Systems Product
One occasionally reads newspaper accounts of how two program-
mers in a remodeled garage have built an important program that
surpasses the best efforts of large teams. And every programmer
is prepared to believe such tales, for he knows that he could build
any program much faster than the 1000 statements/year reported
for industrial teams.
Why then have not all industrial programming teams been
replaced by dedicated garage duos? One must look at what is being
In the upper left of Fig. 1.1 is a program. It is complete in itself,
ready to be run by the author on the system on which it was
developed. That is the thing commonly produced in garages, and
The Programming Systems Product
^^^^^^^^^^^^^^^^^^^Hr ' yo
System Integration) ;
Fig. 1.1 Evolution of the programming systems product
that is the object the individual programmer uses in estimating
There are two ways a program can be converted into a more
useful, but more costly, object. These two ways are represented by
the boundaries in the diagram.
Moving down across the horizontal boundary, a program
becomes a programming product. This is a program that can be run.
The Tar Pit
tested, repaired, and extended by anybody. It is usable in many
operating environments, for many sets of data. To become a gener-
ally usable programming product, a program must be written in a
generalized fashion. In particular the range and form of inputs
must be generalized as much as the basic algorithm will reasonably
allow. Then the program must be thoroughly tested, so that it can
be depended upon. This means that a substantial bank of test
cases, exploring the input range and probing its boundaries, must
be prepared, run, and recorded. Finally, promotion of a program
to a programming product requires its thorough documentation, so
that anyone may use it, fix it, and extend it. As a rule of thumb,
I estimate that a programming product costs at least three times as
much as a debugged program with the same function.
Moving across the vertical boundary, a program becomes a
component in a programming system. This is a collection of interact-
ing programs, coordinated in function and disciplined in format,
so that the assemblage constitutes an entire facility for large tasks.
To become a programming system component, a program must be
written so that every input and output conforms in syntax and
semantics with precisely defined interfaces. The program must
also be designed so that it uses only a prescribed budget of re-
sources — memory space, input-output devices, computer time. Fi-
nally, the program must be tested with other system components,
in all expected combinations. This testing must be extensive, for
the number of cases grows combinatorially. It is time-consuming,
for subtle bugs arise from unexpected interactions of debugged
components. A programming system component costs at least
three times as much as a stand-alone program of the same func-
tion. The cost may be greater if the system has many components.
In the lower right-hand corner of Fig. 1.1 stands the program-
ming systems product. This differs from the simple program in all of
the above ways. It costs nine times as much. But it is the truly
useful object, the intended product of most system programming
The Joys of the Craft
The Joys of the Craft
Why is programming fun? What delights may its practitioner
expect as his reward?
First is the sheer joy of making things. As the child delights
in his mud pie, so the adult enjoys building things, especially
things of his own design. I think this delight must be an image of
God's delight in making things, a delight shown in the distinctness
and newness of each leaf and each snowflake.
Second is the pleasure of making things that are useful to
other people. Deep within, we want others to use our work and
to find it helpful. In this respect the programming system is not
essentially different from the child's first clay pencil holder ''for
Third is the fascination of fashioning complex puzzle-like
objects of interlocking moving parts and watching them work in
subtle cycles, playing out the consequences of principles built in
from the beginning. The programmed computer has all the fasci-
nation of the pinball machine or the jukebox mechanism, carried
to the ultimate.
Fourth is the joy of always learning, which springs from the
nonrepeating nature of the task. In one way or another the prob-
lem is ever new, and its solver learns something: sometimes practi-
cal, sometimes theoretical, and sometimes both.
Finally, there is the delight of working in such a tractable
medium. The programmer, like the poet, works only slightly re-
moved from pure thought-stuff. He builds his castles in the air,
from air, creating by exertion of the imagination. Few media of
creation are so flexible, so easy to polish and rework, so readily
capable of realizing grand conceptual structures. (As we shall see
later, this very tractability has its own problems.)
Yet the program construct, unlike the poet's words, is real in
the sense that it moves and works, producing visible outputs sepa-
rate from the construct itself. It prints results, draws pictures,
produces sounds, moves arms. The magic of myth and legend has
8 The Tar Pit
come true in our time. One types the correct incantation on a
keyboard, and a display screen comes to Hfe, showing things that
never were nor could be.
Programming then is fun because it gratifies creative longings
built deep within us and delights sensibilities we have in common
with all men.
The Woes of the Craft
Not all is delight, however, and knowing the inherent woes makes
it easier to bear them when they appear.
First, one must perform perfectly. The computer resembles the
magic of legend in this respect, too. If one character, one pause, of
the incantation is not strictly in proper form, the magic doesn't
work. Human beings are not accustomed to being perfect, and few
areas of human activity demand it. Adjusting to the requirement
for perfection is, 1 think, the most difficult part of learning to
Next, other people set one's objectives, provide one's re-
sources, and furnish one's information. One rarely controls the
circumstances of his work, or even its goal. In management terms,
one's authority is not sufficient for his responsibility. It seems that
in all fields, however, the jobs where things get done never have
formal authority commensurate with responsibility. In practice,
actual (as opposed to formal) authority is acquired from the very
momentum of accomplishment.
The dependence upon others has a particular case that is espe-
cially painful for the system programmer. He depends upon other
people's programs. These are often maldesigned, poorly imple-
mented, incompletely delivered (no source code or test cases), and
poorly documented. So he must spend hours studying and fixing
things that in an ideal world would be complete, available, and
The next woe is that designing grand concepts is fun; finding
nitty little bugs is just work. With any creative activity come
The Woes of the Craft
dreary hours of tedious, painstaking labor, and programming is no
Next, one finds that debugging has a Unear convergence, or
worse, where one somehow expects a quadratic sort of approach
to the end. So testing drags on and on, the last difficult bugs taking
more time to find than the first.
The last woe, and sometimes the last straw, is that the product
over which one has labored so long appears to be obsolete upon
(or before) completion. Already colleagues and competitors are in
hot pursuit of new and better ideas. Already the displacement of
one's thought-child is not only conceived, but scheduled.
This always seems worse than it really is. The new and better
product is generally not available when one completes his own; it
is only talked about. It, too, will require months of development.
The real tiger is never a match for the paper one, unless actual use
is wanted. Then the virtues of reality have a satisfaction all their
Of course the technological base on which one builds is always
advancing. As soon as one freezes a design, it becomes obsolete in
terms of its concepts. But implementation of real products de-
mands phasing and quantizing. The obsolescence of an implemen-
tation must be measured against other existing implementations,
not against unrealized concepts. The challenge and the mission are
to find real solutions to real problems on actual schedules with
This then is programming, both a tar pit in which many efforts
have floundered and a creative activity with joys and woes all its
own. For many, the joys far outweigh the woes, and for them the
remainder of this book will attempt to lay some boardwalks across
The Mythical Man-Month
Fonde En 1840
AVIS AU PUBLIC
Faire de la bonne cuisine demande un certain temps. Si on vous fait attendre,
c'est pour mieux vous servir, et vous plaire.
Cotelettes d'agneau grillees 2.50
Cotelettes d'agneau aux champignons frais 2.75
Filet de boeuf aux champignons frais 4. 75
Ris de veau a la financiere 2.00
Filet de boeuf nature 3.75
Tournedos Medicis 3.25
Pigeonneaux sauce paradis 3.50
Tournedos sauce bearnaise 3.25
Entrecote minute 2.75
Filet de boeuf bearnaise 4.00
Tripes a la mode de Caen (commander d'avance) 2.00
Entrecote marchand de vin 4.00
Cotelettes d'agneau maison d'or 2.7f
Cotelettes d'agneau a la parisienne 2.!
Fois de volaille a la brochette 1.50
Tournedos nature 2.75
Filet de boeuf a la hawaienne 4.00
Tournedos a la hawaienne 3.25
Tournedos marchand de vin 3.25
Pigeonneaux grilles 3.00
Entrecote nature 3.75
Chateaubriand (30 minutes) 7A
Epinards sauce creme .60 Chou-fleur au gratin .60
Broccoli sauce hollandaise .80 Asperges fraiches au beurre .90
Pommes de terre au gratin .60 Carottes a la creme .60
Haricots verts au berre .60 Pommes de terre souf flees
Petits pois a la franQaise .75
Salade Antoine .60
Salade Mirabeau .75
Salade laitue au roquefort .80
Salade de laitue aux tomates
Salade de legumes .60
Fonds d'artichauts Bayard
Salade de laitue aux oeufs .60
Tomate f rappee a la Jules Cesar .60
.60 Salade de coeur de palmier 1.00
Salade aux pointes d' asperges .60
1 .00 Avocat a la vinaigrette .60
Gateau moka .50
Meringue glacee .60
Crepes Suzette 1.25
Glace sauce chocolat .60
Fruits de saison a I'eau-de-vie . 75
Omelette souf flee a la Jules Cesar (2)
Omelette Alaska Antoine (2) 2.50
Cerises jubile 1.25
Crepes a la gelee .80
Crepes nature . 70
Omelette au rhum 1 . 1
Glace a la vanille .50
Praises au kirsch .9
Peche Melba .6
Liederkranz .50 Gruyere .50
Fromage a la creme Philadelphie .50
Cafe brulot diabolique 1.00
CAFE ET THE
Cafe au lait .20
The glace .20
EAUX MINERALES—BIERE—CIGARES— CIGARETTES
White Rock Biere locale
Vichy Cliquot Club Canada Dry
Ro\? h. Alciatore, Propri^tairc
713*717 Rue St. I^ouis Nouvcllc Orleans, feouisianc
The Mythical ISAan-hAonih
Good cooking takes time. If you are made to wait, it is to
serve j/ou better, and to please you.
MENU OF RESTAURANT ANTOINE, NEW ORLEANS
14 The Mythical Man-Month
More software projects have gone awry for lack of calendar time
than for all other causes combined. Why is this cause of disaster
First, our techniques of estimating are poorly developed. More
seriously, they reflect an unvoiced assumption which is quite un-
true, i.e., that all will go well.
Second, our estimating techniques fallaciously confuse effort
with progress, hiding the assumption that men and months are
Third, because we are uncertain of our estimates, software
managers often lack the courteous stubbornness of Antoine's chef.
Fourth, schedule progress is poorly monitored. Techniques
proven and routine in other engineering disciplines are considered
radical innovations in software engineering.
Fifth, when schedule slippage is recognized, the natural (and
traditional) response is to add manpower. Like dousing a fire with
gasoline, this makes matters worse, much worse. More fire re-
quires more gasoline, and thus begins a regenerative cycle which
ends in disaster.
Schedule monitoring will be the subject of a separate essay.
Let us consider other aspects of the problem in more detail.
All programmers are optimists. Perhaps this modern sorcery espe-
cially attracts those who believe in happy endings and fairy god-
mothers. Perhaps the hundreds of nitty frustrations drive away all
but those who habitually focus on the end goal. Perhaps it is
merely that computers are young, programmers are younger, and
the young are always optimists. But however the selection process
works, the result is indisputable: 'This time it will surely run,'' or
"I just found the last bug."
So the first false assumption that underlies the scheduling of
systems programming is that all will ^o well, i.e., that each task will
take only as lon^ as it "ought" to take.
The pervasiveness of optimism among programmers deserves
more than a flip analysis. Dorothy Sayers, in her excellent book,
The Mind of the Maker, divides creative activity into three stages:
the idea, the implementation, and the interaction. A book, then,
or a computer, or a program comes into existence first as an ideal
construct, built outside time and space, but complete in the mind
of the author. It is realized in time and space, by pen, ink, and
paper, or by wire, silicon, and ferrite. The creation is complete
when someone reads the book, uses the computer, or runs the
program, thereby interacting with the mind of the maker.
This description, which Miss Sayers uses to illuminate not
only human creative activity but also the Christian doctrine of the
Trinity, will help us in our present task. For the human makers of
things, the incompletenesses and inconsistencies of our ideas ,
become clear only during implementation. Thus it is that writing,
experimentation, ''working out'' are essential disciplines for the
In many creative activities the medium of execution is intract-
able. Lumber splits; paints smear; electrical circuits ring. These
physical limitations of the medium constrain the ideas that may
be expressed, and they also create unexpected difficulties in the
Implementation, then, takes time and sweat both because of
the physical media and because of the inadequacies of the under-
lying ideas. We tend to blame the physical media for most of our
implementation difficulties; for the media are not ''ours'' in the
way the ideas are, and our pride colors our judgment.
Computer programming, however, creates with an exceed-
ingly tractable medium. The programmer builds from pure
thought-stuff: concepts and very flexible representations thereof.
Because the medium is tractable, we expect few difficulties in
implementation; hence our pervasive optimism. Because our ideas
are faulty, we have bugs; hence our optimism is unjustified.
In a single task, the assumption that all will go well has a
probabilistic effect on the schedule. It might indeed go as planned,
16 The Mythical Man-Month
for there is a probability distribution for the delay that will be
encountered, and ''no delay" has a finite probability. A large pro-
gramming effort, however, consists of many tasks, some chained
end-to-end. The probability that each will go well becomes van-
The second fallacious thought mode is expressed in the very unit
of effort used in estimating and scheduling: the man-month. Cost
does indeed vary as the product of the number of men and the
number of months. Progress does not. Hence the man-month as a unit
for measuring the size of a job is a dangerous and deceptive myth. It
implies that men and months are interchangeable.
Men and months are interchangeable commodities only when
a task can be partitioned among many workers with no communica-
tion among them (Fig. 2.1). This is true of reaping wheat or picking
cotton; it is not even approximately true of systems programming.
J I 1
Fig. 2.1 Time versus number of workers — perfectly partitionable task
The Man-Month 17
When a task cannot be partitioned because of sequential con-
straints, the appHcation of more effort has no effect on the sched-
ule (Fig. 2.2). The bearing of a child takes nine months, no matter
how many women are assigned. Many software tasks have this
characteristic because of the sequential nature of debugging.
Fig. 2.2 Time versus number of workers — unpartitionable task
In tasks that can be partitioned but which require communica-
tion among the subtasks, the effort of communication must be
added to the amount of work to be done. Therefore the best that
can be done is somewhat poorer than an even trade of men for
months (Fig. 2.3).
18 The Mythical Man-Month
Fig. 2.3 Time versus number of workers — partitionable task requiring
The added burden of communication is made up of two parts,
training and intercommunication. Each worker must be trained in
the technology, the goals of the effort, the overall strategy, and the
plan of work. This training cannot be partitioned, so this part of
the added effort varies linearly with the number of workers.^
Intercommunication is worse. If each part of the task must be
separately coordinated with each other part, the effort increases as
n(n-l)/2. Three workers require three times as much pairwise
intercommunication as two; four require six times as much as two.
If, moreover, there need to be conferences among three, four, etc.,
workers to resolve things jointly, matters get worse yet. The added
effort of communicating may fully counteract the division of the
original task and bring us to the situation of Fig. 2.4.
Systems Test 19
Fig. 2.4 Time versus number of workers — task with complex interrela-
Since software construction is inherently a systems effort — an
exercise in complex interrelationships — communication effort is
great, and it quickly dominates the decrease in individual task time
brought about by partitioning. Adding more men then lengthens,
not shortens, the schedule.
No parts of the schedule are so thoroughly affected by sequential
constraints as component debugging and system test. Further-
more, the time required depends on the number and subtlety of
the errors encountered. Theoretically this number should be zero.
Because of optimism, we usually expect the number of bugs to be
20 The Mythical Man-Month
smaller than it turns out to be. Therefore testing is usually the
most mis-scheduled part of programming.
For some years I have been successfully using the following
rule of thumb for scheduling a software task:
Va component test and early system test
Va system test, all components in hand.
This differs from conventional scheduling in several important
1. The fraction devoted to planning is larger than normal. Even
so, it is barely enough to produce a detailed and solid specifi-
cation, and not enough to include research or exploration of
totally new techniques.
2. The half of the schedule devoted to debugging of completed
code is much larger than normal.
3. The part that is easy to estimate, i.e., coding, is given only
one-sixth of the schedule.
In examining conventionally scheduled projects, I have found
that few allowed one-half of the projected schedule for testing,
but that most did indeed spend half of the actual schedule for that
purpose. Many of these were on schedule until and except in
Failure to allow enough time for system test, in particular, is
peculiarly disastrous. Since the delay comes at the end of the
schedule, no one is aware of schedule trouble until almost the
delivery date. Bad news, late and without warning, is unsettling
to customers and to managers.
Furthermore, delay at this point has unusually severe finan-
cial, as well as psychological, repercussions. The project is fully
staffed, and cost-per-day is maximum. More seriously, the soft-
ware is to support other business effort (shipping of computers,
operation of new facilities, etc.) and the secondary costs of delay-
ing these are very high, for it is almost time for software shipment.
Regenerative Schedule Disaster 21
Indeed, these secondary costs may far outweigh all others. It is
therefore very important to allow enough system test time in the
Observe that for the programmer, as for the chef, the urgency of
the patron may govern the scheduled completion of the task, but
it cannot govern the actual completion. An omelette, promised in
two minutes, may appear to be progressing nicely. But when it has
not set in two minutes, the customer has two choices — wait or eat
it raw. Software customers have had the same choices.
The cook has another choice; he can turn up the heat. The
result is often an omelette nothing can save — burned in one part,
raw in another.
Now I do not think software managers have less inherent
courage and firmness than chefs, nor than other engineering man-
agers. But false scheduling to match the patron's desired date is
much more common in our discipline than elsewhere in engineer-
ing. It is very difficult to make a vigorous, plausible, and job-
risking defense of an estimate that is derived by no quantitative
method, supported by little data, and certified chiefly by the
hunches of the managers.
Clearly two solutions are needed. We need to develop and
publicize productivity figures, bug-incidence figures, estimating
rules, and so on. The whole profession can only profit from sharing
Until estimating is on a sounder basis, individual managers
will need to stiffen their backbones and defend their estimates
with the assurance that their poor hunches are better than wish-
Regenerative Schedule Disaster
What does one do when an essential software project is behind
schedule? Add manpower, naturally. As Figs. 2.1 through 2.4 sug-
gest, this may or may not help.
22 The Mythical Man-Month
Let us consider an example.^ Suppose a task is estimated at 12
man-months and assigned to three men for four months, and that
there are measurable mileposts A, B, C, D, which are scheduled to
fall at the end of each month (Fig. 2,5).
Now suppose the first milepost is not reached until two
months have elapsed (Fig. 2.6). What are the alternatives facing
1. Assume that the task must be done on time. Assume that only
the first part of the task was misestimated, so Fig. 2.6 tells the
story accurately. Then 9 man-months of effort remain, and
two months, so AVi men will be needed. Add 2 men to the 3
2. Assume that the task must be done on time. Assume that the
whole estimate was uniformly low, so that Fig. 2.7 really
describes the situation. Then 18 man-months of effort remain,
and two months, so 9 men will be needed. Add 6 men to the
3 4 5
Regenerative Schedule Disaster 23
1 month delay
(9 man/months remain)
1 1 1 1 1
3 4 E
(18 m/m remain
3 4 5
24 The Mythical Man-Month
3. Reschedule. I like the advice given by P. Fagg, an experienced
hardware engineer, 'Take no small slips/' That is, allow
enough time in the new schedule to ensure that the work can
be carefully and thoroughly done, and that rescheduling will
not have to be done again.
4. Trim the task. In practice this tends to happen anyway, once
the team observes schedule slippage. Where the secondary
costs of delay are very high, this is the only feasible action.
The manager's only alternatives are to trim it formally and
carefully, to reschedule, or to watch the task get silently
trimmed by hasty design and incomplete testing.
In the first two cases, insisting that the unaltered task be
completed in four months is disastrous. Consider the regenerative
effects, for example, for the first alternative (Fig. 2.8). The two new
men, however competent and however quickly recruited, will re-
quire training in the task by one of the experienced men. If this
takes a month, 3 man-months will have been devoted to work not in the
original estimate. Furthermore, the task, originally partitioned three
ways, must be repartitioned into ^we parts; hence some work
already done will be lost, and system testing must be lengthened.
So at the end of the third month, substantially more than 7 man-
months of effort remain, and 5 trained people and one month are
available. As Fig. 2.8 suggests, the product is just as late as if no
one had been added (Fig. 2.6).
To hope to get done in four months, considering only training
time and not repartitioning and extra systems test, would require
adding 4 men, not 2, at the end of the second month. To cover
repartitioning and system test effects, one would have to add still
other men. Now, however, one has at least a 7-man team, not a
3-man one; thus such aspects as team organization and task divi-
sion are different in kind, not merely in degree.
Notice that by the end of the third month things look very
black. The March 1 milestone has not been reached in spite of all
Regenerative Schedule Disaster 25
for 7+ m/m
the managerial effort. The temptation is very strong to repeat the
cycle, adding yet more manpower. Therein lies madness.
The foregoing assumed that only the first milestone was
misestimated. If on March 1 one makes the conservative assump-
tion that the whole schedule was optimistic, as Fig. 2.7 depicts, one
wants to add 6 men just to the original task. Calculation of the
training, repartitioning, system testing effects is left as an exercise
for the reader. Without a doubt, the regenerative disaster will
yield a poorer product, later, than would rescheduling with the
original three men, unaugmented.
Oversimplifying outrageously, we state Brooks's Law:
Adding manpower to a late software project makes it later.
This then is the demythologizing of the man-month. The
number of months of a project depends upon its sequential con-
26 The Mythical Man-Month
straints. The maximum number of men depends upon the number
of independent subtasks. From these two quantities one can derive
schedules using fewer men and more months. (The only risk is
product obsolescence.) One cannot, however, get workable sched-
ules using more men and fewer months. More software projects
have gone awry for lack of calendar time than for all other causes
The Surgical Team
The Surgical Team
These studies revealed large individual differences between
high and low performers, often by an order of magnitude.
SACKMAN. ERIKSON, AND GRANT
30 The Surgical Team
At computer society meetings one continually hears young pro-
gramming managers assert that they favor a small, sharp team of
first-class people, rather than a project with hundreds of program-
mers, and those by implication mediocre. So do we all.
But this naive statement of the alternatives avoids the hard
problem — how does one build large systems on a meaningful
schedule? Let us look at each side of this question in more detail.
Programming managers have long recognized wide productivity
variations between good programmers and poor ones. But the
actual measured magnitudes have astounded all of us. In one of
their studies, Sackman, Erikson, and Grant were measuring perfor-
mances of a group of experienced programmers. Within just this
group the ratios between best and worst performances averaged
about 10:1 on productivity measurements and an amazing 5:1 on
program speed and space measurements! In short the $20,000/year
programmer may well be 10 times as productive as the
$10,000/year one. The converse may be true, too. The data
showed no correlation whatsoever between experience and per-
formance. (I doubt if that is universally true.)
I have earlier argued that the sheer number of minds to be
coordinated affects the cost of the effort, for a major part of the
cost is communication and correcting the ill effects of miscom-
munication (system debugging). This, too, suggests that one wants
the system to be built by as few minds as possible. Indeed, most
experience with large programming systems shows that the brute-
force approach is costly, slow, inefficient, and produces systems
that are not conceptually integrated. OS/360, Exec 8, Scope 6600,
Multics, TSS, SAGE, etc. — the list goes on and on.
The conclusion is simple: if a 200-man project has 25 manag-
ers who are the most competent and experienced programmers,
fire the 175 troops and put the managers back to programming.
The Problem 31
Now let's examine this solution. On the one hand, it fails to
approach the ideal of the small sharp team, which by common
consensus shouldn't exceed 10 people. It is so large that it will need
to have at least two levels of management, or about five managers.
It will additionally need support in finance, personnel, space, sec-
retaries, and machine operators.
On the other hand, the original 200-man team was not large
enough to build the really large systems by brute-force methods.
Consider OS/360, for example. At the peak over 1000 people were
working on it — programmers, writers, machine operators, clerks,
secretaries, managers, support groups, and so on. From 1963
through 1966 probably 5000 man-years went into its design, con-
struction, and documentation. Our postulated 200-man team
would have taken 25 years to have brought the product to its
present stage, if men and months traded evenly!
This then is the problem with the small, sharp team concept:
it is too slow for really big systems. Consider the OS/360 job as it
might be tackled with a small, sharp team. Postulate a 10-man
team. As a bound, let them be seven times as productive as medi-
ocre programmers in both programming and documentation, be-
cause they are sharp. Assume OS/360 was built only by mediocre
programmers (which is far from the truth). As a bound, assume
that another productivity improvement factor of seven comes
from reduced communication on the part of the smaller team.
Assume the same team stays on the entire job. Well, 5000/(10 X
7X7) = 10; they can do the 5000 man-year job in 10 years. Will
the product be interesting 10 years after its initial design? Or will
it have been made obsolete by the rapidly developing software
The dilemma is a cruel one. For efficiency and conceptual
integrity, one prefers a few good minds doing design and construc-
tion. Yet for large systems one wants a way to bring considerable
manpower to bear, so that the product can make a timely appear-
ance. How can these two needs be reconciled?
32 The Surgical Team
A proposal by Harlan Mills offers a fresh and creative solu-
tion.^''' Mills proposes that each segment of a large job be tackled
by a team, but that the team be organized like a surgical team
rather than a hog-butchering team. That is, instead of each mem-
ber cutting away on the problem, one does the cutting and the
others give him every support that will enhance his effectiveness
A little thought shows that this concept meets the desiderata,
if it can be made to work. Few minds are involved in design and
construction, yet many hands are brought to bear. Can it work?
Who are the anesthesiologists and nurses on a programming team,
and how is the work divided? Let me freely mix metaphors to
suggest how such a team might work if enlarged to include all
The surgeon. Mills calls him a chief programmer. He personally
defines the functional and performance specifications, designs the
program, codes it, tests it, and writes its documentation. He writes
in a structured programming language such as PL/I, and has effec-
tive access to a computing system which not only runs his tests but
also stores the various versions of his programs, allows easy file
updating, and provides text editing for his documentation. He
needs great talent, ten years experience, and considerable systems
and application knowledge, whether in applied mathematics,
business data handling, or whatever.
The copilot. He is the alter ego of the surgeon, able to do any
part of the job, but is less experienced. His main function is to
share in the design as a thinker, discussant, and evaluator. The
surgeon tries ideas on him, but is not bound by his advice. The
copilot often represents his team in discussions of function and
interface with other teams. He knows all the code intimately. He
researches alternative design strategies. He obviously serves as
insurance against disaster to the surgeon. He may even write code,
but he is not responsible for any part of the code.
Mills's Proposal 33
The administrator. The surgeon is boss, and he must have the
last word on personnel, raises, space, and so on, but he must spend
almost none of his time on these matters. Thus he needs a profes-
sional administrator who handles money, people, space, and ma-
chines, and who interfaces with the administrative machinery of
the rest of the organization. Baker suggests that the administrator
has a full-time job only if the project has substantial legal, con-
tractual, reporting, or financial requirements because of the user-
producer relationship. Otherwise, one administrator can serve two
The editor. The surgeon is responsible for generating the docu-
mentation — for maximum clarity he must write it. This is true of
both external and internal descriptions. The editor, however, takes
the draft or dictated manuscript produced by the surgeon and
criticizes it, reworks it, provides it with references and bibliogra-
phy, nurses it through several versions, and oversees the mechan-
ics of production.
Two secretaries. The administrator and the editor will each need
a secretary; the administrator's secretary will handle project corre-
spondence and non-product files.
The program clerk. He is responsible for maintaining all the
technical records of the team in a programming-product library.
The clerk is trained as a secretary and has responsibility for both
machine-readable and human-readable files.
All computer input goes to the clerk, who logs and keys it if
required. The output listings go back to him to be filed and in-
dexed. The most recent runs of any model are kept in a status
notebook; all previous ones are filed in a chronological archive.
Absolutely vital to Mills's concept is the transformation of
programming "from private art to public practice" by making all
the computer runs visible to all team members and identifying all
programs and data as team property, not private property.
The specialized function of the program clerk relieves pro-
grammers of clerical chores, systematizes and ensures proper per-
34 The Surgical Team
formance of those oft-neglected chores, and enhances the team's
most valuable asset — its work-product. Clearly the concept as set
forth above assumes batch runs. When interactive terminals are
used, particularly those with no hard-copy output, the program
clerk's functions do not diminish, but they change. Now he logs
all updates of team program copies from private working copies,
still handles all batch runs, and uses his own interactive facility to
control the integrity and availability of the growing product.
The toolsmith. File-editing, text-editing, and interactive debug-
ging services are now readily available, so that a team will rarely
need its own machine and machine-operating crew. But these
services must be available with unquestionably satisfactory re-
sponse and reliability; and the surgeon must be sole judge of the
adequacy of the service available to him. He needs a toolsmith,
responsible for ensuring this adequacy of the basic service and for
constructing, maintaining, and upgrading special tools — mostly
interactive computer services — needed by his team. Each team will
need its own toolsmith, regardless of the excellence and reliability
of any centrally provided service, for his job is to see to the tools
needed or wanted by his surgeon, without regard to any other
team's needs. The tool-builder will often construct specialized
utilities, catalogued procedures, macro libraries.
The tester. The surgeon will need a bank of suitable test cases
for testing pieces of his work as he writes it, and then for testing
the whole thing. The tester is therefore both an adversary who
devises system test cases from the functional specs, and an assis-
tant who devises test data for the day-by-day debugging. He
would also plan testing sequences and set up the scaffolding re-
quired for component tests.
The language lawyer. By the time Algol came along, people
began to recognize that most computer installations have one or
two people who delight in mastery of the intricacies of a program-
ming language. And these experts turn out to be very useful and
very widely consulted. The talent here is rather different from that
of the surgeon, who is primarily a system designer and who thinks
How It Works 35
representations. The language lawyer can find a neat and efficient
way to use the language to do difficult, obscure, or tricky things.
Often he will need to do small studies (two or three days) on good
technique. One language lawyer can service two or three surgeons.
This, then, is how 10 people might contribute in well-
differentiated and specialized roles on a programming team built
on the surgical model.
How It Works
The team just defined meets the desiderata in several ways. Ten
people, seven of them professionals, are at work on the problem,
but the system is the product of one mind — or at most two, acting
Notice in particular the differences between a team of two
programmers conventionally organized and the surgeon-copilot
team. First, in the conventional team the partners divide the work,
and each is responsible for design and implementation of part of
the work. In the surgical team, the surgeon and copilot are each
cognizant of all of the design and all of the code. This saves the
labor of allocating space, disk accesses, etc. It also ensures the
conceptual integrity of the work.
Second, in the conventional team the partners are equal, and
the inevitable differences of judgment must be talked out or com-
promised. Since the work and resources are divided, the differ-
ences in judgment are confined to overall strategy and interfacing,
but they are compounded by differences of interest — e.g., whose
space will be used for a buffer. In the surgical team, there are no
differences of interest, and differences of judgment are settled by
the surgeon unilaterally. These two differences — lack of division
of the problem and the superior-subordinate relationship — make
it possible for the surgical team to act uno animo.
Yet the specialization of function of the remainder of the team
is the key to its efficiency, for it permits a radically simpler com-
munication pattern among the members, as Fig. 3.1 shows.
36 The Surgical Team
Fig. 3.1 Communication patterns in 10-man programming teams
Baker's article^ reports on a single, small-scale test of the team
concept. It worked as predicted for that case, with phenomenally
So far, so good. The problem, however, is how to build things that
today take 5000 man-years, not things that take 20 or 30. A 10-
man team can be effective no matter how it is organized, if the
whole job is within its purview. But how is the surgical team
concept to be used on large jobs when several hundred people are
brought to bear on the task?
The success of the scaling-up process depends upon the fact
that the conceptual integrity of each piece has been radically im-
proved — that the number of minds determining the design has
Scaling Up 37
been divided by seven. So it is possible to put 200 people on a
problem and face the problem of coordinating only 20 minds,
those of the surgeons.
For that coordination problem, however, separate techniques
must be used, and these are discussed in succeeding chapters. Let
it suffice here to say that the entire system also must have concep-
tual integrity, and that requires a system architect to design it all,
from the top down. To make that job manageable, a sharp distinc-
tion must be made between architecture and implementation, and
the system architect must confine himself scrupulously to archi-
tecture. However, such roles and techniques have been shown to
be feasible and, indeed, very productive.
and System Design
and System Design
This great church is an incomparable work of art. There is
neither aridity nor confusion in the tenets it sets forth. . . .
// is the zenith of a style, the work of artists who had
understood and assimilated all their predecessors ' successes,
in complete possession of the techniques of their times, but
using them without indiscreet display nor gratuitous feats
It was Jean d'Orbais who undoubtedly conceived the
general plan of the building, a plan which was respected,
at least in its essential elements, by his successors. This is
one of the reasons for the extreme coherence and unity of
REIMS CATHEDRAL GUIDEBOOK'
Photographies Emmanuel Boudot-Lamotte
42 Aristocracy, Democracy, and System Design
Most European cathedrals show differences in plan or architec-
tural style between parts built in different generations by different
builders. The later builders were tempted to ''improve'' upon the
designs of the earlier ones, to reflect both changes in fashion and
differences in individual taste. So the peaceful Norman transept
abuts and contradicts the soaring Gothic nave, and the result pro-
claims the pridefulness of the builders as much as the glory of
Against these, the architectural unity of Reims stands in glori-
ous contrast. The joy that stirs the beholder comes as much from
the integrity of the design as from any particular excellences. As
the guidebook tells, this integrity was achieved by the self-abne-
gation of eight generations of builders, each of whom sacrificed
some of his ideas so that the whole might be of pure design. The
result proclaims not only the glory of God, but also His power to
salvage fallen men from their pride.
Even though they have not taken centuries to build, most
programming systems reflect conceptual disunity far worse than
that of cathedrals. Usually this arises not from a serial succession
of master designers, but from the separation of design into many
tasks done by many men.
I will contend that conceptual integrity is the most important
consideration in system design. It is better to have a system omit
certain anomalous features and improvements, but to reflect one
set of design ideas, than to have one that contains many good but
independent and uncoordinated ideas. In this chapter and the next
two, we will examine the consequences of this theme for program-
ming system design:
• How is conceptual integrity to be achieved?
• Does not this argument imply an elite, or aristocracy of archi-
tects, and a horde of plebeian implementers whose creative
talents and ideas are suppressed?
Achieving Conceptual Integrity 43
• How does one keep the architects from drifting off into the
blue with unimplementable or costly specifications?
• How does one ensure that every trifling detail of an architec-
tural specification gets communicated to the implementer,
properly understood by him, and accurately incorporated into
Achieving Conceptual Integrity
The purpose of a programming system is to make a computer easy
to use. To do this, it furnishes languages and various facilities that
are in fact programs invoked and controlled by language features.
But these facilities are bought at a price: the external description
of a programming system is ten to twenty times as large as the
external description of the computer system itself. The user finds
it far easier to specify any particular function, but there are far
more to choose from, and far more options and formats to remem-
Ease of use is enhanced only if the time gained in functional
specification exceeds the time lost in learning, remembering, and
searching manuals. With modern programming systems this gain
does exceed the cost, but in recent years the ratio of gain to cost
seems to have fallen as more and more complex functions have
been added. I am haunted by the memory of the ease of use of the
IBM 650, even without an assembler or any other software at all.
Because ease of use is the purpose, this ratio of function to
conceptual complexity is the ultimate test of system design. Nei-
ther function alone nor simplicity alone defines a good design.
This point is widely misunderstood. Operating System/360 is
hailed by its builders as the finest ever built, because it indisputa-
bly has the most function. Function, and not simplicity, has al-
ways been the measure of excellence for its designers. On the
other hand, the Time-Sharing System for the PDP-10 is hailed by
its builders as the finest, because of its simpHcity and the spareness
44 Aristocracy, Democracy, and System Design
of its concepts. By any measure, however, its function is not even
in the same class as that of OS/360. As soon as ease of use is held
up as the criterion, each of these is seen to be unbalanced, reaching
for only half of the true goal.
For a given level of function, however, that system is best in
which one can specify things with the most simplicity and
straightforwardness. Simplicity is not enough. Mooers's TRAC
language and Algol 68 achieve simplicity as measured by the num-
ber of distinct elementary concepts. They are not, however,
straightforward. The expression of the things one wants to do often
requires involuted and unexpected combinations of the basic facil-
ities. It is not enough to learn the elements and rules of combina-
tion; one must also learn the idiomatic usage, a whole lore of how
the elements are combined in practice. Simplicity and straightfor-
wardness proceed from conceptual integrity. Every part must re-
flect the same philosophies and the same balancing of desiderata.
Every part must even use the same techniques in syntax and
analogous notions in semantics. Ease of use, then, dictates unity
of design, conceptual integrity.
Aristocracy and Democracy
Conceptual integrity in turn dictates that the design must proceed
from one mind, or from a very small number of agreeing resonant
Schedule pressures, however, dictate that system building
needs many hands. Two techniques are available for resolving this
dilemma. The first is a careful division of labor between architec-
ture and implementation. The second is the new way of structur-
ing programming implementation teams discussed in the previous
The separation of architectural effort from implementation is
a very powerful way of getting conceptual integrity on very large
projects. I myself have seen it used with great success on IBM's
Stretch computer and on the System/360 computer product line.
Aristocracy and Democracy 45
I have seen it fail through lack of application on Operating Sys-
By the architecture of a system, I mean the complete and de-
tailed specification of the user interface. For a computer this is the
programming manual. For a compiler it is the language manual. For
a control program it is the manuals for the language or languages
used to invoke its functions. For the entire system it is the union
of the manuals the user must consult to do his entire job.
The architect of a system, like the architect of a building, is
the user's agent. It is his job to bring professional and technical
knowledge to bear in the unalloyed interest of the user, as opposed
to the interests of the salesman, the fabricator, etc.^
Architecture must be carefully distinguished from implemen-
tation. As Blaauw has said, ''Where architecture tells what hap-
pens, implementation tells how it is made to happen."^ He gives
as a simple example a clock, whose architecture consists of the
face, the hands, and the winding knob. When a child has learned
this architecture, he can tell time as easily from a wristwatch as
from a church tower. The implementation, however, and its real-
ization, describe what goes on inside the case — powering by any
of many mechanisms and accuracy control by any of many.
In System/360, for example, a single computer architecture is
implemented quite differently in each of some nine models. Con-
versely, a single implementation, the Model 30 data flow, memory,
and microcode, serves at different times for four different architec-
tures: a System/360 computer, a multiplex channel with up to 224
logically independent subchannels, a selector channel, and a 1401
The same distinction is equally applicable to programming
systems. There is a U.S. standard Fortran IV. This is the architec-
ture for many compilers. Within this architecture many imple-
mentations are possible: text-in-core or compiler-in-core,
fast-compile or optimizing, syntax-directed or ad-hoc. Likewise
any assembler language or job-control language admits of many
implementations of the assembler or scheduler.
46 Aristocracy, Democracy, and System Design
Now we can deal with the deeply emotional question of aris-
tocracy versus democracy. Are not the architects a new aristocracy,
an intellectual elite, set up to tell the poor dumb implementers
what to do? Has not all the creative work been sequestered for this
elite, leaving the implementers as cogs in the machine? Won't one
get a better product by getting the good ideas from all the team,
following a democratic philosophy, rather than by restricting the
development of specifications to a few?
As to the last question, it is the easiest. I will certainly not
contend that only the architects will have good architectural ideas.
Often the fresh concept does come from an implementer or from
a user. However, all my own experience convinces me, and I have
tried to show, that the conceptual integrity of a system determines
its ease of use. Good features and ideas that do not integrate with
a system's basic concepts are best left out. If there appear many
such important but incompatible ideas, one scraps the whole sys-
tem and starts again on an integrated system with different basic
As to the aristocracy charge, the answer must be yes and no.
Yes, in the sense that there must be few architects, their product
must endure longer than that of an implementer, and the architect
sits at the focus of forces which he must ultimately resolve in the
user's interest. If a system is to have conceptual integrity, someone
must control the concepts. That is an aristocracy that needs no
No, because the setting of external specifications is not more
creative work than the designing of implementations. It is just
different creative work. The design of an implementation, given an
architecture, requires and allows as much design creativity, as
many new ideas, and as much technical brilliance as the design of
the external specifications. Indeed, the cost-performance ratio of
the product will depend most heavily on the implementer, just as
ease of use depends most heavily on the architect.
There are many examples from other arts and crafts that lead
one to believe that discipline is good for art. Indeed, an artist's
What Does the Implementer Do While Waiting? 47
aphorism asserts, 'Torm is liberating/' The worst buildings are
those whose budget was too great for the purposes to be served.
Bach's creative output hardly seems to have been squelched by the
necessity of producing a limited-form cantata each week. I am sure
that the Stretch computer would have had a better architecture
had it been more tightly constrained; the constraints imposed by
the System/360 Model 30's budget were in my opinion entirely
beneficial for the Model 75's architecture.
Similarly, I observe that the external provision of an architec-
ture enhances, not cramps, the creative style of an implementing
group. They focus at once on the part of the problem no one has
addressed, and inventions begin to flow. In an unconstrained im-
plementing group, most thought and debate goes into architectural
decisions, and implementation proper gets short shrift.^
This effect, which I have seen many times, is confirmed by
R. W. Conway, whose group at Cornell built the PL/C compiler
for the PL/I language. He says, ''We finally decided to implement
the language unchanged and unimproved, for the debates about
language would have taken all our effort."^
What Does the Implementer Do While Waiting?
It is a very humbling experience to make a multimillion-dollar
mistake, but it is also very memorable. I vividly recall the night
we decided how to organize the actual writing of external specifi-
cations for OS/360. The manager of architecture, the manager of
control program implementation, and I were threshing out the
plan, schedule, and division of responsibilities.
The architecture manager had 10 good men. He asserted that
they could write the specifications and do it right. It would take
ten months, three more than the schedule allowed.
The control program manager had 150 men. He asserted that
they could prepare the specifications, with the architecture team
coordinating; it would be well-done and practical, and he could do
it on schedule. Furthermore, if the architecture team did it, his 150
men would sit twiddling their thumbs for ten months.
48 Aristocracy, Democracy, and System Design
To this the architecture manager responded that if I gave the
control program team the responsibiUty, the result would not in
fact be on time, but would also be three months late, and of much
lower quality. I did, and it was. He was right on both counts.
Moreover, the lack of conceptual integrity made the system far
more costly to build and change, and I would estimate that it
added a year to debugging time.
Many factors, of course, entered into that mistaken decision;
but the overwhelming one was schedule time and the appeal of
putting all those 150 implementers to work. It is this siren song
whose deadly hazards I would now make visible.
When it is proposed that a small architecture team in fact
write all the external specifications for a computer or a program-
ming system, the implementers raise three objections:
• The specifications will be too rich in function and will not
reflect practical cost considerations.
• The architects will get all the creative fun and shut out the
inventiveness of the implementers.
• The many implementers will have to sit idly by while the
specifications come through the narrow funnel that is the
The first of these is a real danger, and it will be treated in the
next chapter. The other two are illusions, pure and simple. As we
have seen above, implementation is also a creative activity of the
first order. The opportunity to be creative and inventive in imple-
mentation is not significantly diminished by working within a
given external specification, and the order of creativity may even
be enhanced by that discipline. The total product will surely be.
The last objection is one of timing and phasing. A quick an-
swer is to refrain from hiring implementers until the specifications
are complete. This is what is done when a building is constructed.
In the computer systems business, however, the pace is
quicker, and one wants to compress the schedule as much as
possible. How much can specification and building be overlapped?
What Does the Implementer Do While Waiting? 49
As Blaauw points out, the total creative effort involves three
distinct phases: architecture, implementation, and realization. It
turns out that these can in fact be begun in parallel and proceed
In computer design, for example, the implementer can start as
soon as he has relatively vague assumptions about the manual,
somewhat clearer ideas about the technology, and well-defined
cost and performance objectives. He can begin designing data
flows, control sequences, gross packaging concepts, and so on. He
devises or adapts the tools he will need, especially the record-
keeping system, including the design automation system.
Meanwhile, at the realization level, circuits, cards, cables,
frames, power supplies, and memories must each be designed,
refined, and documented. This work proceeds in parallel with
architecture and implementation.
The same thing is true in programming system design. Long
before the external specifications are complete, the implementer
has plenty to do. Given some rough approximations as to the
function of the system that will be ultimately embodied in the
external specifications, he can proceed. He must have well-defined
space and time objectives. He must know the system configuration
on which his product must run. Then he can begin designing
module boundaries, table structures, pass or phase breakdowns,
algorithms, and all kinds of tools. Some time, too, must be spent
in communicating with the architect.
Meanwhile, on the realization level there is much to be done
also. Programming has a technology, too. If the machine is a new
one, much work must be done on subroutine conventions, super-
visory techniques, searching and sorting algorithms.^
Conceptual integrity does require that a system reflect a single
philosophy and that the specification as seen by the user flow from
a few minds. Because of the real division of labor into architecture,
implementation, and realization, however, this does not imply that
a system so designed will take longer to build. Experience shows
the opposite, that the integral system goes together faster and
50 Aristocracy, Democracy, and System Design
takes less time to test. In effect, a widespread horizontal division
of labor has been sharply reduced by a vertical division of labor,
and the result is radically simplified communications and im-
proved conceptual integrity.
The Second-System Effect
^A- ^ ,
. :- 1
The Second-System Effect
Adde parvum parvo magnus acervus erit.
[Add little to little and there will be a big pile. ]
Turning house for air traffic. Lithograph, Paris, 1882
The Bettman Archive
54 The Second-System Effect
If one separates responsibility for functional specification from
responsibility for building a fast, cheap product, what discipline
bounds the architect's inventive enthusiasm?
The fundamental answer is thoroughgoing, careful, and sym-
pathetic communication between architect and builder. Neverthe-
less there are finer-grained answers that deserve attention.
Interactive Discipline for the Architect
The architect of a building works against a budget, using estimat-
ing techniques that are later confirmed or corrected by the con-
tractors' bids. It often happens that all the bids exceed the budget.
The architect then revises his estimating technique upward and his
design downward for another iteration. He may perhaps suggest
to the contractors ways to implement his design more cheaply
than they had devised.
An analogous process governs the architect of a computer
system or a programming system. He has, however, the advantage
of getting bids from the contractor at many early points in his
design, almost any time he asks for them. He usually has the
disadvantage of working with only one contractor, who can raise
or lower his estimates to reflect his pleasure with the design. In
practice, early and continuous communication can give the archi-
tect good cost readings and the builder confidence in the design
without blurring the clear division of responsibilities.
The architect has two possible answers when confronted with
an estimate that is too high: cut the design or challenge the esti-
mate by suggesting cheaper implementations. This latter is inher-
ently an emotion-generating activity. The architect is now
challenging the builder's way of doing the builder's job. For it to
be successful, the architect must
• remember that the builder has the inventive and creative re-
sponsibility for the implementation; so the architect suggests,
Self-Discipline — The Second-System Effect 55
• always be prepared to suggest a way of implementing any-
thing he specifies, and be prepared to accept any other way
that meets the objectives as well;
• deal quietly and privately in such suggestions;
• be ready to forego credit for suggested improvements.
Normally the builder will counter by suggesting changes to
the architecture. Often he is right — some minor feature may have
unexpectedly large costs when the implementation is worked out.
Self-Discipline — The Second-System Effect
An architect's first work is apt to be spare and clean. He knows he
doesn't know what he's doing, so he does it carefully and with
As he designs the first work, frill after frill and embellishment
after embellishment occur to him. These get stored away to be
used ''next time." Sooner or later the first system is finished, and
the architect, with firm confidence and a demonstrated mastery of
that class of systems, is ready to build a second system.
This second is the most dangerous system a man ever designs.
When he does his third and later ones, his prior experiences will
confirm each other as to the general characteristics of such sys-
tems, and their differences will identify those parts of his experi-
ence that are particular and not generalizable.
The general tendency is to over-design the second system,
using all the ideas and frills that were cautiously sidetracked on
the first one. The result, as Ovid says, is a "big pile." For example,
consider the IBM 709 architecture, later embodied in the 7090.
This is an upgrade, a second system for the very successful and
clean 704. The operation set is so rich and profuse that only about
half of it was regularly used.
Consider as a stronger case the architecture, implementation,
and even the realization of the Stretch computer, an outlet for the
56 The Second-System Effect
pent-up inventive desires of many people, and a second system for
most of them. As Strachey says in a review:
I get the impression that Stretch is in some way the end of one line
of development. Like some early computer programs it is immensely
ingenious, immensely complicated, and extremely effective, but some-
how at the same time crude, wasteful, and inelegant, and one feels
that there must be a better way of doing things. ^
Operating System/360 was the second system for most of its
designers. Groups of its designers came from building the 1410-
7010 disk operating system, the Stretch operating system, the
Project Mercury real-time system, and IBSYS for the 7090. Hardly
anyone had experience with two previous operating systems.^ So
OS/360 is a prime example of the second- system effect, a Stretch
of the software art to which both the commendations and the
reproaches of Strachey's critique apply unchanged.
For example, OS/360 devotes 26 bytes of the permanently
resident date-turnover routine to the proper handling of Decem-
ber 31 on leap years (when it is Day 366). That might have been
left to the operator.
The second-system effect has another manifestation some-
what different from pure functional embellishment. That is a ten-
dency to refine techniques whose very existence has been made
obsolete by changes in basic system assumptions. OS/360 has
many examples of this.
Consider the linkage editor, designed to load separately-com-
piled programs and resolve their cross-references. Beyond this
basic function it also handles program overlays. It is one of the
finest overlay facilities ever built. It allows overlay structuring to
be done externally, at linkage time, without being designed into
the source code. It allows the overlay structure to be changed from
run to run without recompilation. It furnishes a rich variety of
useful options and facilities. In a sense it is the culmination of
years of development of static overlay technique.
Self-Discipline— The Second-System Effect 57
Yet it is also the last and finest of the dinosaurs, for it belongs
to a system in which multiprogramn\ing is the normal mode and
dynamic core allocation the basic assumption. This is in direct
conflict with the notion of using static overlays. How much better
the system would work if the efforts devoted to overlay manage-
ment had been spent on making the dynamic core allocation and
the dynamic cross-referencing facilities really fast!
Furthermore, the linkage editor requires so much space and
itself contains many overlays that even when it is used just for
linkage without overlay management, it is slower than most of the
system compilers. The irony of this is that the purpose of the
linker is to avoid recompilation. Like a skater whose stomach gets
ahead of his feet, refinement proceeded until the system assump-
tions had been quite outrun.
The TESTRAN debugging facility is another example of this
tendency. It is the culmination of batch debugging facilities, fur-
nishing truly elegant snapshot and core dump capabilities. It uses
the control section concept and an ingenious generator technique
to allow selective tracing and snapshotting without interpretive
overhead or recompilation. The imaginative concepts of the Share
Operating System^ for the 709 have been brought to full bloom.
Meanwhile, the whole notion of batch debugging without
recompilation was becoming obsolete. Interactive computing sys-
tems, using language interpreters or incremental compilers have
provided the most fundamental challenge. But even in batch sys-
tems, the appearance of fast-compile/slow-execute compilers has
made source-level debugging and snapshotting the preferred tech-
nique. How much better the system would have been if the TES-
TRAN effort had been devoted instead to building the interactive
and fast-compile facilities earlier and better!
Yet another example is the scheduler, which provides truly
excellent facilities for managing a fixed-batch job stream. In a real
sense, this scheduler is the refined, improved, and embellished
second system succeeding the 1410-7010 Disk Operating System,
58 The Second-System Effect
a batch system unmultiprogrammed except for input-output and
intended chiefly for business appHcations. As such, the OS/360
scheduler is good. But it is almost totally uninfluenced by the
OS/360 needs of remote job entry, multiprogramming, and per-
manently resident interactive subsystems. Indeed, the scheduler's
design makes these hard.
How does the architect avoid the second-system effect? Well,
obviously he can't skip his second system. But he can be conscious
of the peculiar hazards of that system, and exert extra self-disci-
pline to avoid functional ornamentation and to avoid extrapola-
tion of functions that are obviated by changes in assumptions and
A discipline that will open an architect's eyes is to assign each
little function a value: capability x is worth not more than m bytes
of memory and n microseconds per invocation. These values will
guide initial decisions and serve during implementation as a guide
and warning to all.
How does the project manager avoid the second-system
effect? By insisting on a senior architect who has at least two
systems under his belt. Too, by staying aware of the special temp-
tations, he can ask the right questions to ensure that the philo-
sophical concepts and objectives are fully reflected in the detailed
Passing the Ward
i# d: :M
■0' M 'P^ ^ . ^IS
O- m^ «:. Q :^
Ov ^ o o & a ^
o m. a m.
^ 0, o 1:1 o
o o a 43 c3 a
Passing the Word
He'll sit here and he'll say, ''Do this! Do that!" And
nothing will happen.
HARRYS. TRUMAN, ON PRESIDENTIAL POWER'
"The Seven Trumpets" from The Wells Apocalypse, 14th century
The Bettman Archive
62 Passing the Word
Assuming that he has the disciplined, experienced architects and
that there are many implementers, how shall the manager ensure
that everyone hears, understands, and implements the architects'
decisions? How can a group of 10 architects maintain the concep-
tual integrity of a system which 1000 men are building? A whole
technology for doing this was worked out for the System/360
hardware design effort, and it is equally applicable to software
Written Specifications — the Manual
The manual, or written specification, is a necessary tool, though
not a sufficient one. The manual is the external specification of the
product. It describes and prescribes every detail of what the user
sees. As such, it is the chief product of the architect.
Round and round goes its preparation cycle, as feedback from
users and implementers shows where the design is awkward to use
or build. For the sake of implementers it is important that the
changes be quantized — that there be dated versions appearing on
The manual must not only describe everything the user does
see, including all interfaces; it must also refrain from describing
what the user does not see. That is the implementer's business, and
there his design freedom must be unconstrained. The architect
must always be prepared to show an implementation for any
feature he describes, but he must not attempt to dictate the imple-
The style must be precise, full, and accurately detailed. A user
will often refer to a single definition, so each one must repeat all
the essentials and yet all must agree. This tends to make manuals
dull reading, but precision is more important than liveliness.
The unity of System/360's Principles of Operation springs from
the fact that only two pens wrote it: Gerry Blaauw's and Andris
Padegs'. The ideas are those of about ten men, but the casting of
those decisions into prose specifications must be done by only one
Formal Definitions 63
or two, if the consistency of prose and product is to be maintained.
For the writing of a definition will necessitate a host of mini-
decisions which are not of full-debate importance. An example in
System/360 is the detail of how the Condition Code is set after
each operation. Not trivial, however, is the principle that such
mini-decisions be made consistently throughout.
I think the finest piece of manual writing I have ever seen is
Blaauw's Appendix to System/ 360 Principles of Operation. This de-
scribes with care and precision the limits of System/360 compati-
bility. It defines compatibility, prescribes what is to be achieved,
and enumerates those areas of external appearance where the ar-
chitecture is intentionally silent and where results from one model
may differ from those of another, where one copy of a given model
may differ from another copy, or where a copy may differ even
from itself after an engineering change. This is the level of preci-
sion to which manual writers aspire, and they must define what
is not prescribed as carefully as what is.
English, or any other human language, is not naturally a precision
instrument for such definitions. Therefore the manual writer must
strain himself and his language to achieve the precision needed.
An attractive alternative is to use a formal notation for such defini-
tions. After all, precision is the stock in trade, the raison d'etre of
Let us examine the merits and weaknesses of formal defini-
tions. As noted, formal definitions are precise. They tend to be
complete; gaps show more conspicuously, so they are filled sooner.
What they lack is comprehensibility. With English prose one can
show structural principles, delineate structure in stages or levels,
and give examples. One can readily mark exceptions and empha-
size contrasts. Most important, one can explain why. The formal
definitions put forward so far have inspired wonder at their ele-
gance and confidence in their precision. But they have demanded
64 Passing the Word
prose explanations to make their content easy to learn and teach.
For these reasons, I think we will see future specifications to con-
sist of both a formal definition and a prose definition.
An ancient adage warns, ''Never go to sea with two chronom-
eters; take one or three.'' The same thing clearly applies to prose
and formal definitions. If one has both, one must be the standard,
and the other must be a derivative description, clearly labeled as
such. Either can be the primary standard. Algol 68 has a formal
definition as standard and a prose definition as descriptive. PL/I
has the prose as standard and the formal description as derivative.
System/360 also has prose as standard with a derived formal de-
Many tools are available for formal definition. The Backus-
Naur Form is familiar for language definition, and it is amply
discussed in the literature.^ The formal description of PL/I uses
new notions of abstract syntax, and it is adequately described.^
Iverson's APL has been used to describe machines, most notably
the IBM 7090^ and System/360.^
Bell and Newell have proposed new notations for describing
both configurations and machine architectures, and they have il-
lustrated these with several machines, including the DEC PDP-8,®
the 7090,^ and System/360.''
Almost all formal definitions turn out to embody or describe
an implementation of the hardware or software system whose
externals they are prescribing. Syntax can be described without
this, but semantics are usually defined by giving a program that
carries out the defined operation. This is of course an implementa-
tion, and as such it over-prescribes the architecture. So one must
take care to indicate that the formal definition applies only to
externals, and one must say what these are.
Not only is a formal definition an implementation, an imple-
mentation can serve as a formal definition. When the first compat-
ible computers were built, this was exactly the technique used.
The new machine was to match an existing machine. The manual
was vague on some points? "Ask the machine!" A test program
Formal Definitions 65
would be devised to determine the behavior, and the new machine
would be built to match.
A programmed simulator of a hardware or software system
can serve in precisely the same way. It is an implementation; it
runs. So all questions of definition can be resolved by testing it.
Using an implementation as a definition has some advantages.
All questions can be settled unambiguously by experiment. De-
bate is never needed, so answers are quick. Answers are always as
precise as one wants, and they are always correct, by definition.
Opposed to these one has a formidable set of disadvantages. The
implementation may over-prescribe even the externals. Invalid
syntax always produces some result; in a policed system that result
is an invalidity indication and nothing more. In an unpoliced system
all kinds of side effects may appear, and these may have been used
by programmers. When we undertook to emulate the IBM 1401
on System/360, for example, it developed that there were 30
different ''curios'' — side effects of supposedly invalid operations —
that had come into widespread use and had to be considered as
part of the definition. The implementation as a definition overpre-
scribed; it not only said what the machine must do, it also said a
great deal about how it had to do it.
Then, too, the implementation will sometimes give unex-
pected and unplanned answers when sharp questions are asked,
and the de facto definition will often be found to be inelegant in
these particulars precisely because they have never received any
thought. This inelegance will often turn out to be slow or costly
to duplicate in another implementation. For example, some ma-
chines leave trash in the multiplicand register after a multiplica-
tion. The precise nature of this trash turns out to be part of the
de facto definition, yet duplicating it may preclude the use of a
faster multiplication algorithm.
Finally, the use of an implementation as a formal definition is
peculiarly susceptible to confusion as to whether the prose de-
scription or the formal description is in fact the standard. This is
especially true of programmed simulations. One must also refrain
66 Passing the Word
from modifications to the implementation while it is serving as a
A lovely technique for disseminating and enforcing definitions, is
available for the software system architect. It is especially useful
for establishing the syntax, if not the semantics, of intermodule
interfaces. This technique is to design the declaration of the passed
parameters or shared storage, and to require the implementations
to include that declaration via a compile-time operation (a macro
or a % INCLUDE in PL/I). If, in addition, the whole interface is
referenced only by symbolic names, the declaration can be
changed by adding or inserting new variables with only recompi-
lation, not alteration, of the using program.
Conferences and Courts
Needless to say, meetings are necessary. The hundreds of man-to-
man consultations must be supplemented by larger and more for-
mal gatherings. We found two levels of these to be useful. The first
is a weekly half-day conference of all the architects, plus official
representatives of the hardware and software implementers, and
the market planners. The chief system architect presides.
Anyone can propose problems or changes, but proposals are
usually distributed in writing before the meeting. A new problem
is usually discussed a while. The emphasis is on creativity, rather
than merely decision. The group attempts to invent many solu-
tions to problems, then a few solutions are passed to one or more
of the architects for detailing into precisely worded manual change
Detailed change proposals then come up for decisions. These
have been circulated and carefully considered by implementers
and users, and the pros and cons are well delineated. If a consensus
emerges, well and good. If not, the chief architect decides. Minutes
Conferences and Courts 67
are kept and decisions are formally, promptly, and widely dis-
Decisions from the weekly conferences give quick results and
allow work to proceed. If anyone is too unhappy, instant appeals
to the project manager are possible, but this happens very rarely.
The fruitfulness of these meetings springs from several
1. The same group — architects, users, and implementers — meets
weekly for months. No time is needed for bringing people up
2. The group is bright, resourceful, well versed in the issues, and
deeply involved in the outcome. No one has an ''advisory"
role. Everyone is authorized to make binding commitments.
3. When problems are raised, solutions are sought both within
and outside the obvious boundaries.
4. The formality of written proposals focuses attention, forces
decision, and avoids committee-drafted inconsistencies.
5. The clear vesting of decision-making power in the chief archi-
tect avoids compromise and delay.
As time goes by, some decisions don't wear well. Some minor
matters have never been wholeheartedly accepted by one or an-
other of the participants. Other decisions have developed unfore-
seen problems, and sometimes the weekly meeting didn't agree to
reconsider these. So there builds up a backlog of minor appeals,
open issues, or disgruntlements. To settle these we held annual
supreme court sessions, lasting typically two weeks. (I would hold
them every six months if I were doing it again.)
These sessions were held just before major freeze dates for the
manual. Those present included not only the architecture group
and the programmers' and implementers' architectural representa-
tives, but also the managers of programming, marketing, and im-
plementation efforts. The System/360 project manager presided.
The agenda typically consisted of about 200 items, mostly minor,
which were enumerated in charts placarded around the room. All
68 Passing the Word
sides were heard and decisions made. By the miracle of computer-
ized text editing (and lots of fine staff work), each participant
found an updated manual, embodying yesterday's decisions, at his
seat every morning.
These ''fall festivals" were useful not only for resolving deci-
sions, but also for getting them accepted. Everyone was heard,
everyone participated, everyone understood better the intricate
constraints and interrelationships among decisions.
System/360 architects had two almost unprecedented advantages:
enough time to work carefully, and political clout equal to that of
the implementers. The provision of enough time came from the
schedule of the new technology; the political equality came from
the simultaneous construction of multiple implementations. The
necessity for strict compatibility among these served as the best
possible enforcing agent for the specifications.
In most computer projects there comes a day when it is discov-
ered that the machine and the manual don't agree. When the
confrontation follows, the manual usually loses, for it can be
changed far more quickly and cheaply than the machine. Not so,
however, when there are multiple implementations. Then the de-
lays and costs associated with fixing the errant machine can be
overmatched by delays and costs in revising the machines that
followed the manual faithfully.
This notion can be fruitfully applied whenever a programming
language is being defined. One can be certain that several inter-
preters or compilers will sooner or later have to be built to meet
various objectives. The definition will be cleaner and the discipline
tighter if at least two implementations are built initially.
The Telephone Log
As implementation proceeds, countless questions of architectural
interpretation arise, no matter how precise the specification. Obvi-
Product Test 69
ously many such questions require amplifications and clarifica-
tions in the text. Others merely reflect misunderstandings.
It is essential, however, to encourage the puzzled implementer
to telephone the responsible architect and ask his question, rather
than to guess and proceed. It is just as vital to recognize that the
answers to such questions are ex cathedra architectural pronounce-
ments that must be told to everyone.
One useful mechanism is a telephone log kept by the architect.
In it he records every question and every answer. Each week the
logs of the several architects are concatenated, reproduced, and
distributed to the users and implementers. While this mechanism
is quite informal, it is both quick and comprehensive.
The project manager's best friend is his daily adversary, the inde-
pendent product-testing organization. This group checks ma-
chines and programs against specifications and serves as a devil's
advocate, pinpointing every conceivable defect and discrepancy.
Every development organization needs such an independent tech-
nical auditing group to keep it honest.
In the last analysis the customer is the independent auditor.
In the merciless light of real use, every flaw will show. The prod-
uct-testing group then is the surrogate customer, specialized for
finding flaws. Time after time, the careful product tester will find
places where the word didn't get passed, where the design deci-
sions were not properly understood or accurately implemented.
For this reason such a testing group is a necessary link in the chain
by which the design word is passed, a link that needs to operate
early and simultaneously with design.
Why Did theTower
of Babel Fail?
Why Did theTower
of Babel Fail?
Now the whole earth used only one language, with few
words. On the occasion of a migration from the east, men
discovered a plain in the land of Shinar, and settled there.
Then they said to one another, ''Come, let us make bricks,
burning them well. " So they used bricks for stone, and
bitumen for mortar. Then they said, "Come, let us build
ourselves a city with a tower whose top shall reach the
heavens (thus making a name for ourselves), so that we
may not be scattered all over the earth. " Then the Lord
came down to look at the city and tower which human
beings had built. The Lord said, "They are just one people,
and they all have the same language. If this is what they
can do as a beginning, then nothing that they resolve to do
will be impossible for them. Come, let us go down, and
there make such a babble of their language that they will
not understand one another's speech. " Thus the Lord
dispersed them from there all over the earth, so that they
had to stop building the city.
P. Breughel, the Elder, "Turmbau zu Babel," 1563
Kunsthistorisches Museum, Vienna
74 Why Did The Tower of Babel Fail?
A Management Audit of the Babel Project
According to the Genesis account, the tower of Babel was man's
second major engineering undertaking, after Noah's ark. Babel
was the first engineering fiasco.
The story is deep and instructive on several levels. Let us,
however, examine it purely as an engineering project, and see what
management lessons can be learned. How well was their project
equipped with the prerequisites for success? Did they have:
1. A clear mission? Yes, although naively impossible. The project
failed long before it ran into this fundamental limitation.
2. Manpower? Plenty of it.
3. Materials? Clay and asphalt are abundant in Mesopotamia.
4. Enough time? Yes, there is no hint of any time constraint.
5. Adequate technology? Yes, the pyramidal or conical structure
is inherently stable and spreads the compressive load well.
Clearly masonry was well understood. The project failed be-
fore it hit technological limitations.
Well, if they had all of these things, why did the project fail?
Where did they lack? In two respects — communication, and its con-
sequent, organization. They were unable to talk with each other;
hence they could not coordinate. When coordination failed, work
ground to a halt. Reading between the lines we gather that lack
of communication led to disputes, bad feelings, and group jeal-
ousies. Shortly the clans began to move apart, preferring isolation
Communication in the Large Programming Project
So it is today. Schedule disaster, functional misfits, and system
bugs all arise because the left hand doesn't know what the right
hand is doing. As work proceeds, the several teams slowly change
the functions, sizes, and speeds of their own programs, and they
explicitly or implicitly change their assumptions about the inputs
available and the uses to be made of the outputs.
The Project Workbook 75
For example, the implementer of a program-overlaying func-
tion may run into problems and reduce speed, relying on statistics
that show how rarely this function will arise in application pro-
grams. Meanwhile, back at the ranch, his neighbor may be design-
ing a major part of the supervisor so that it critically depends upon
the speed of this function. This change in speed itself becomes a
major specification change, and it needs to be proclaimed abroad
and weighed from a system point of view.
How, then, shall teams communicate with one another? In as
many ways as possible.
• Informally. Good telephone service and a clear definition of
intergroup dependencies will encourage the hundreds of calls
upon which common interpretation of written documents de-
• Meetings. Regular project meetings, with one team after an-
other giving technical briefings, are invaluable. Hundreds of
minor misunderstandings get smoked out this way.
• Workbook. A formal project workbook must be started at the
beginning. This deserves a section by itself.
The Project Workbook
What. The project workbook is not so much a separate docu-
ment as it is a structure imposed on the documents that the project
will be producing anyway.
All the documents of the project need to be part of this struc-
ture. This includes objectives, external specifications, interface
specifications, technical standards, internal specifications, and ad-
Why. Technical prose is almost immortal. If one examines the
genealogy of a customer manual for a piece of hardware or soft-
ware, one can trace not only the ideas, but also many of the very
sentences and paragraphs back to the first memoranda proposing
the product or explaining the first design. For the technical writer,
the paste-pot is as mighty as the pen.
76 Why Did The Tower of Babel Fail?
Since this is so, and since tomorrow's product-quality manuals
will grow from today's memos, it is very important to get the
structure of the documentation right. The early design of the
project workbook ensures that the documentation structure itself
is crafted, not haphazard. Moreover, the establishment of a struc-
ture molds later writing into segments that fit into that structure.
The second reason for the project workbook is control of the
distribution of information. The problem is not to restrict infor-
mation, but to ensure that relevant information gets to all the
people who need it.
The first step is to number all memoranda, so that ordered lists
of titles are available and each worker can see if he has what he
wants. The organization of the workbook goes well beyond this
to establish a tree-structure of memoranda. The tree-structure
allows distribution hsts to be maintained by subtree, if that is
Mechanics. As with so many programming management prob-
lems, the technical memorandum problem gets worse nonlinearly
as size increases. With 10 people, documents can simply be num-
bered. With 100 people, several linear sequences will often suffice.
With 1000, scattered inevitably over several physical locations, the
need for a structured workbook increases and the size of the work-
book increases. How then shall the mechanics be handled?
I think this was well done on the OS/360 project. The need
for a well-structured workbook was strongly urged by O. S.
Locken, who had seen its effectiveness on his previous project, the
1410-7010 operating system.
We quickly decided that each programmer should see all the
material, i.e., should have a copy of the workbook in his own
Of critical importance is timely updating. The workbook must
be current. This is very difficult to do if whole documents must be
retyped for changes. In a looseleaf book, however, only pages need
to be changed. We had available a computer-driven text-editing
system, and this proved invaluable for timely maintenance. Offset
The Project Workbook 11
masters were prepared directly on the computer printer, and
turnaround time was less than a day. The recipient of all these
updated pages has an assimilation problem, however. When he
first receives a changed page, he wants to know, ''What has been
changed?'' When he later consults it, he wants to know, "What is
the definition today?"
The latter need is met by the continually maintained docu-
ment. Highlighting of changes requires other steps. First, one must
mark changed text on the page, e.g., by a vertical bar in the margin
alongside every altered line. Second, one needs to distribute with
the new pages a short, separately written change summary that
lists the changes and remarks on their significance.
Our project had not been under way six months before we hit
another problem. The workbook was about five feet thick! If we
had stacked up the 100 copies serving programmers in our offices
in Manhattan's Time-Life Building, they would have towered
above the building itself. Furthermore, the daily change distribu-
tion averaged two inches, some 150 pages to be interfiled in the
whole. Maintenance of the workbook began to take a significant
time from each workday.
At this point we switched to microfiche, a change that saved
a million dollars, even allowing for the cost of a microfiche reader
for each office. We were able to arrange excellent turnaround on
microfiche production; the workbook shrank from three cubic feet
to one-sixth of a cubic foot and, most significantly, updates ap-
peared in hundred-page chunks, reducing the interfiling problem
Microfiche has its drawbacks. From the manager's point of
view the awkward interfiling of paper pages ensured that the
changes were read, which was the purpose of the workbook. Mi-
crofiche would make workbook maintenance too easy, unless the
update fiche are distributed with a paper document enumerating
Also, a microfiche cannot readily be highlighted, marked, and
commented by the reader. Documents with which the reader has
78 Why Did The Tower of Babel Fail?
interacted are more effective for the author and more useful for the
On balance I think the microfiche was a very happy mecha-
nism, and 1 would recommend it over a paper workbook for very
How would one do it today? With today's system technology
available, I think the technique of choice is to keep the workbook
on the direct-access file, marked with change bars and revision
dates. Each user would consult it from a display terminal (type-
writers are too slow). A change summary, prepared daily, would
be stored in LIFO fashion at a fixed access point. The programmer
would probably read that daily, but if he missed a day he would
need only read longer the next day. As he read the change sum-
mary, he could interrupt to consult the changed text itself.
Notice that the workbook itself is not changed. It is still the
assemblage of all project documentation, structured according to
a careful design. The only change is in the mechanics of distribu-
tion and consultation. D. C. Engelbart and his colleagues at the
Stanford Research Institute have built such a system and are using
it to build and maintain documentation for the ARPA network.
D. L. Parnas of Carnegie-Mellon University has proposed a
still more radical solution.^ His thesis is that the programmer is
most effective if shielded from, rather than exposed to the details
of construction of system parts other than his own. This presup-
poses that all interfaces are completely and precisely defined.
While that is definitely sound design, dependence upon its perfect
accomplishment is a recipe for disaster. A good information sys-
tem both exposes interface errors and stimulates their correction.
Organization in the Large Programming Project
If there are n workers on a project, there are (n^-n)/2 interfaces
across which there may be communication, and there are poten-
tially almost 2" teams within which coordination must occur. The
purpose of organization is to reduce the amount of communication
Organization in the Large Programming Project 79
and coordination necessary; hence organization is a radical attack
on the communication problems treated above.
The means by which communication is obviated are division of
labor and specialization of function. The tree-like structure of orga-
nizations reflects the diminishing need for detailed communica-
tion when division and specialization of labor are applied.
In fact, a tree organization really arises as a structure of au-
thority and responsibility. The principle that no man can serve
two masters dictates that the authority structure be tree-like. But
the communication structure is not so restricted, and the tree is a
barely passable approximation to the communication structure,
which is a network. The inadequacies of the tree approximation
give rise to staff groups, task forces, committees, and even the
matrix-type organization used in many engineering laboratories.
Let us consider a tree-like programming organization, and
examine the essentials which any subtree must have in order to be
effective. They are:
1. a mission
2. a producer
3. a technical director or architect
4. a schedule
5. a division of labor
6. interface definitions among the parts
All of this is obvious and conventional except the distinction
between the producer and the technical director. Let us first con-
sider the two roles, then their relationship.
What is the role of the producer? He assembles the team,
divides the work, and establishes the schedule. He acquires and
keeps on acquiring the necessary resources. This means that a
major part of his role is communication outside the team, upwards
and sideways. He establishes the pattern of communication and
reporting within the team. Finally, he ensures that the schedule is
met, shifting resources and organization to respond to changing
80 Why Did The Tower of Babel Fail?
How about the technical director? He conceives of the design
to be built, identifies its subparts, specifies how it will look from
outside, and sketches its internal structure. He provides unity and
conceptual integrity to the whole design; thus he serves as a limit
on system complexity. As individual technical problems arise, he
invents solutions for them or shifts the system design as required.
He is, in Al Capp's lovely phrase, ''inside-man at the skunk
works.'' His communications are chiefly within the team. His
work is almost completely technical.
Now it is clear that the talents required for these two roles are
quite different. Talents come in many different combinations; and
the particular combination embodied in the producer and the di-
rector must govern the relationship between them. Organizations
must be designed around the people available; not people fitted
into pure-theory organizations.
Three relationships are possible, and all three are found in
The producer and the technical director may be the same man.
This is readily workable on very small teams, perhaps three to six
programmers. On larger projects it is very rarely workable, for two
reasons. First, the man with strong management talent and strongs
technical talent is rarely found. Thinkers are rare; doers are rarer;
and thinker-doers are rarest. ^
Second, on the larger project each of the roles is necessarily a
full-time job, or more. It is hard for the producer to delegate
enough of his duties to give him any technical time. It is impossi-
ble for the director to delegate his without compromising the
conceptual integrity of the design.
The producer may be boss, the director his right-hand man.
The difficulty here is to establish the director's authoriti/ to make
technical decisions without impacting his time as would putting
him in the management chain-of-command.
Obviously the producer must proclaim the director's technical
authority, and he must back it in an extremely high proportion of
Organization in the Large Programming Project 81
the test cases that will arise. For this to be possible, the producer
and the director must see alike on fundamental technical philoso-
phy; they must talk out the main technical issues privately, before
they really become timely; and the producer must have a high
respect for the director's technical prowess.
Less obviously, the producer can do all sorts of subtle things
with the symbols of status (office size, carpet, furnishing, carbon
copies, etc.) to proclaim that the director, although outside the
management line, is a source of decision power.
This can be made to work very effectively. Unfortunately it
is rarely tried. The job done least well by project managers is to
utilize the technical genius who is not strong on management
The director may be boss, and the producer his right-hand man.
Robert Heinlein, in The Man Who Sold the Moon, describes such
an arrangement in a graphic for-instance:
Coster buried his face in his hands, then looked up. "I know it. I know
what needs to be done — but every time I try to tackle a technical
problem some bloody fool wants me to make a decision about trucks
— or telephones — or some damn thing. I'm sorry, Mr. Harriman.
I thought I could do it. "
Harriman said very gently, "Don 't let it throw you. Bob. You
haven 't had much sleep lately, have you? Tell you what — we'll put
over a fast one on Ferguson. Til take that desk you 're at for a few
days and build you a set-up to protect you against such things. I want
that brain of yours thinking about reaction vectors and fuel efficiencies
and design stresses, not about contracts for trucks. "Harriman stepped
to the door, looked around the outer office and spotted a man who might
or might not be the office's chief clerk. "Hey you! C'mere. "
The man looked startled, got up, came to the door and said, "Yes?"
"I want that desk in the corner and all the stuff that's on it moved
to an empty office on this floor, right away. "
82 Why Did The Tower of Babel Fail?
He supervised getting Coster and his other desk moved into another
office, saw to it that the phone in the new office was disconnected, and,
as an afterthought, had a couch moved in there, too. "We'll install
a projector, and a drafting machine and bookcases and other junk like
that tonight, " he told Coster. "Just make a list of anything you need
— to work on engineering/' He went hack to the nominal chief -
engineer 's office and got happily to work trying to figure where the
organization stood and what was wrong with it.
Some four hours later he took Berkeley in to meet Coster. The chief
engineer was asleep at his desk, head cradled on his arms. Harriman
started to hack out, hut Coster roused. "Oh! Sorry, " he said, blush-
ing, "I must have dozed off. "
"TTiat's why I brought you the couch, " said Harriman. "It's more
restful. Bob, meet Jock Berkeley. He 's your new slave. You remain
chief engineer and top, undisputed boss. Jock is Lord High Everything
Else. From now on you 've got absolutely nothing to worry about —
except for the little detail of building a A/foon ship. "
They shook hands. "Just one thing I ask, Mr. Coster, "Berkeley said
seriously, "bypass me all you want to — you'll have to run the
technical show — but for God 's sake record it so Ell know what s going
on. Em going to have a switch placed on your desk that will operate
a sealed recorder at my desk. "
'Tine!" Coster was looking, Harriman thought, younger already.
"And if you want something that is not technical, don 't do it yourself.
Just flip a switch and whistle; it'll get done!" Berkeley glanced at
Harriman. "Ehe Boss says he wants to talk with you about the real
job. Ell leave you and get busy. " He left.
Harriman sat down; Coster followed suit and said, "Whew!"
"I like the looks of that fellow Berkeley. "
Organization in the Large Programming Project 83
"That's good; he's your hvin brother from now on. Stop worrying;
I've used him before. You 'II think you 're living in a well-run hospi-
This account hardly needs any analytic commentary. This
arrangement, too, can be made to work effectively.
I suspect that the last arrangement is best for small teams, as
discussed in Chapter 3, 'The Surgical Team.'' I think the producer
as boss is a more suitable arrangement for the larger subtrees of
a really big project.
The Tower of Babel was perhaps the first engineering fiasco,
but it was not the last. Communication and its consequent, orga-
nization, are critical for success. The techniques of communication
and organization demand from the manager much thought and as
much experienced competence as the software technology itself.
Calling the Shot
Calling the Shot
Practice is the best of all instructors.
Experience is a dear teacher, hut fools will learn at no
POOR RICHARDS ALMANAC
Douglas Crockwell, "Ruth calls his shot/' World Series, 1932
Esquire Magazine and the National Baseball Museum
88 Calling the Shot
How long will a system programming job take? How much effort
will be required? How does one estimate?
I have earlier suggested ratios that seem to apply to planning
time, coding, component test, and system test. First, one must say
that one does not estimate the entire task by estimating the coding
portion only and then applying the ratios. The coding is only
one-sixth or so of the problem, and errors in its estimate or in the
ratios could lead to ridiculous results.
Second, one must say that data for building isolated small
programs are not applicable to programming systems products. For
a program averaging about 3200 words, for example, Sackman,
Erikson, and Grant report an average code-plus-debug time of
about 178 hours for a single programmer, a figure which would
extrapolate to give an annual productivity of 35,800 statements
per year. A program half that size took less than one-fourth as
long, and extrapolated productivity is almost 80,000 statements
per year.^ Planning, documentation, testing, system integration,
and training times must be added. The linear extrapolation of such
sprint figures is meaningless. Extrapolation of times for the hun-
dred-yard dash shows that a man can run a mile in under three
Before dismissing them, however, let us note that these num-
bers, although not for strictly comparable problems, suggest that
effort goes as a power of size even when no communication is
involved except that of a man with his memories.
Figure 8.1 tells the sad story. It illustrates results reported from
a study done by Nanus and Farr^ at System Development Corpo-
ration. This shows an exponent of 1.5; that is,
effort = (constant) X (number of instructions)^^.
Another SDC study reported by Weinwurm^ also shows an expo-
nent near 1.5.
A few studies on programmer productivity have been made,
and several estimating techniques have been proposed. Morin has
prepared a survey of the published data."* Here I shall give only a
few items that seem especially illuminating.
100 200 300 400 500 600 700
Thousands of machine instructions
Fig. 8.1 Programming effort as a function of program size
Charles Portman, manager of ICL's Software Division, Computer
Equipment Organization (Northwest) at Manchester, offers an-
other useful personal insight.^
He found his programming teams missing schedules by about
one-half — each job was taking approximately twice as long as
estimated. The estimates were very careful, done by experienced
teams estimating man-hours for several hundred subtasks on a
PERT chart. When the slippage pattern appeared, he asked them
to keep careful daily logs of time usage. These showed that the
estimating error could be entirely accounted for by the fact that
his teams were only realizing 50 percent of the working week as
actual programming and debugging time. Machine downtime,
higher-priority short unrelated jobs, meetings, paperwork, com-
90 Calling the Shot
pany business, sickness, personal time, etc. accounted for the rest.
In short, the estimates made an unrealistic assumption about the
number of technical work hours per man-year. My own experi-
ence quite confirms his conclusion.®
Joel Aron, manager of Systems Technology at IBM in Gaithers-
burg, Maryland, has studied programmer productivity when
working on nine large systems (briefly, large means more than 25
programmers and 30,000 deliverable instructions).^ He divides
such systems according to interactions among programmers (and-
system parts) and finds productivities as follows:
Very few interactions 10,000 instructions per man-year
Some interactions 5,000
Many interactions 1,500
The man-years do not include support and system test activi-
ties, only design and programming. When these figures are diluted
by a factor of two to cover system test, they closely match Hair's
John Harr, manager of programming for the Bell Telephone Labo-
ratories' Electronic Switching System, reported his and others'
experience in a paper at the 1969 Spring Joint Computer Confer-
ence.® These data are shown in Figs. 8.2, 8.3, and 8.4.
Of these. Fig. 8.2 is the most detailed and the most useful. The
first two jobs are basically control programs; the second two are
basically language translators. Productivity is stated in terms of
debugged words per man-year. This includes programming, com-
ponent test, and system test. It is not clear how much of the
planning effort, or effort in machine support, writing, and the like,
Fig. 8.2 Summary of four No. 1 ESS program jobs
The productivities likewise fall into two classifications; those
for control programs are about 600 words per man-year; those for
translators are about 2200 words per man-year. Note that all four
programs are of similar size — the variation is in size of the work
groups, length of time, and number of modules. Which is cause
and which is effect? Did the control programs require more people
because they were more complicated? Or did they require more
modules and more man-months because they were assigned more
people? Did they take longer because of the greater complexity,
or because more people were assigned? One can't be sure. The
control programs were surely more complex. These uncertainties
aside, the numbers describe the real productivities achieved on a
large system, using present-day programming techniques. As such
they are a real contribution.
Figures 8.3 and 8.4 show some intei-esting data on program-
ming and debugging rates as compared to predicted rates.
92 Calling the Shot
Mar Jun Sep Dec
Fig. 8.3 ESS predicted and actual programming rates
Mar Jun Sep Dec Mar Jun Sep Dec
Fig. 8.4 ESS predicted and actual debugging rates
Corbato's Data 93
IBM OS/360 experience, while not available in the detail of Harris
data, confirms it. Productivities in range of 600-800 debugged
instructions per man-year were experienced by control program
groups. Productivities in the 2000-3000 debugged instructions per
man-year were achieved by language translator groups. These
include planning done by the group, coding component test, sys-
tem test, and some support activities. They are comparable to
Harr's data, so far as I can tell.
Aron's data, Harr's data, and the OS/360 data all confirm ,
striking differences in productivity related to the complexity and j
difficulty of the task itself. My guideline in the morass of estimat-
ing complexity is that compilers are three times as bad as normal j
batch application programs, and operating systems are three times
as bad as compilers.^
Both Harris data and OS/360 data are for assembly language pro-
gramming. Little data seem to have been published on system
programming productivity using higher-level languages. Corbato
of MIT's Project MAC reports, however, a mean productivity of
1200 lines of debugged PL/I statements per man-year on the
MULTICS system (between 1 and 2 million words). ^^
This number is very exciting. Like the other projects, MUL-
TICS includes control programs and language translators. Like the
others, it is producing a system programming product, tested and
documented. The data seem to be comparable in terms of kind of
effort included. And the productivity number is a good average
between the control program and translator productivities of other
But Corbato's number is lines per man-year, not words] Each
statement in his system corresponds to about three to five words
of handwritten code! This suggests two important conclusions.
94 Calling the Shot
Productivity seems constant in terms of elementary state-
ments, a conclusion that is reasonable in terms of the thought
a statement requires and the errors it may include. ^^
Programming productivity may be increased as much as five
times when a suitable high-level language is used.^^
in a Five-Pound Sack
in a Five-Pound Sack
The author should gaze at Noah, and . . . learn, as they
did in the Ark, to crowd a great deal of matter into a very
SYDNEY SMITH, EDINBURGH REVIEW
Engraved from a painting by Heywood Hardy
The Bettman Archive
98 Ten Pounds in a Five-Pound Sack
Program Space as Cost
How big is it? Aside from running time, the space occupied by a
program is a principal cost. This is true even for proprietary pro-
grams, where the user pays the author a fee that is essentially a
share of the development cost. Consider the IBM APL interactive
software system. It rents for $400 per month and, when used, takes
at least 160 K bytes of memory. On a Model 165, memory rents
for about $12 per kilobyte per month. If the program is available
full-time, one pays $400 software rent and $1920 memory rent for
using the program. If one uses the APL system only four hours a
day, the costs are $400 software rent and $320 memory rent per
One frequently hears horror expressed that a 2 M byte ma-
chine may have 400 K devoted to its operating system. This is as
foolish as criticizing a Boeing 747 because it costs $27 million. One
must also ask, ''What does it do?" What does one get in ease-of-
use and in performance (via efficient system utilization) for the
dollars so spent? Could the $4800 per month thus invested in
memory rental have been more fruitfully spent for other hard-
ware, for programmers, for application programs?
The system designer puts part of his total hardware resource
into resident-program memory when he thinks it will do more for
the user in that form than as adders, disks, etc. To do otherwise
would be grossly irresponsible. And the result must be judged as
a whole. No one can criticize a programming system for size per se
and at the same time consistently advocate closer integration of
hardware and software design.
Since size is such a large part of the user cost of a programming
system product, the builder must set size targets, control size, and
devise size-reduction techniques, just as the hardware builder sets
component-count targets, controls component count, and devises
count-reduction techniques. Like any cost, size itself is not bad,
but unnecessary size is.
Size Control 99
For the project manager, size control is partly a technical job and
partly a managerial one. One has to study users and their applica-
tions to set the sizes of the systems to be offered. Then these
systems have to be subdivided, and each component given a size
target. Since size-speed trade-offs come in rather big quantum
jumps, setting size targets is a tricky business, requiring knowl-
edge of the available trade-offs within each piece. The wise man-
ager also saves himself a kitty, to be allocated as work proceeds.
In OS/360, even though all of this was done very carefully,
still other lessons had to be painfully learned.
First, setting size targets for core is not enough; one has to
budget all aspects of size. In most previous operating systems,
systems residence had been on tape, and the long search times of
tape meant that one was not tempted to use it casually to bring
in program segments. OS/360 was disk-resident, like its immedi-
ate predecessors, the Stretch Operating System and the 1410-7010
Disk Operating System. Its builders rejoiced in the freedom of
cheap disk accesses. The initial result was disastrous to perfor-
In setting core sizes for each component, we had not simulta-
neously set access budgets. As anyone with 20-20 hindsight
would expect, a programmer who found his program slopping
over his core target broke it into overlays. This process in itself
added to the total size and slowed execution down. Most seri-
ously, our management control system neither measured nor
caught this. Each man reported as to how much core he was using,
and since he was within target, no one worried.
Fortunately, there came a day early in the effort when the
OS/360 performance simulator began to work. The first result
indicated deep trouble. Fortran H, on a Model 65 with drums,
simulated compiling at five statements per minute! Digging-in
showed that the control program modules were each making
100 Ten Pounds in a Five-Pound Sack
many, many disk accesses. Even high-frequency supervisor
modules were making many trips to the vs^ell, and the result was
quite analogous to page thrashing.
The first moral is clear: Set total size budgets as well as resi-
dent-space budgets; set budgets on backing-store accesses as well
as on sizes.
The next lesson was very similar. The space budgets were set
before precise functional allocations were made to each module.
As a result, any programmer in size trouble examined his code to
see what he could throw over the fence into a neighbor's space.
So buffers managed by the control program became part of the
user's space. More seriously, so did all kinds of control blocks, and
the effect was utterly compromising to the security and protection
of the system.
So the second moral is also clear: Define exactly what a
module must do when you specify how big it must be.
A third and deeper lesson shows through these experiences.
The project was large enough and management communication,
poor enough to prompt many members of the team to see them-
selves as contestants making brownie points, rather than as build-
ers making programming products. Each suboptimized his piece to
meet his targets; few stopped to think about the total effect on the
customer. This breakdown in orientation and communication is a
major hazard for large projects. All during implementation, the
system architects must maintain continual vigilance to ensure con-
tinued system integrity. Beyond this policing mechanism, how-
ever, lies the matter of attitude of the implementers themselves.
Fostering a total- system, user-oriented attitude may well be the
most important function of the programming manager.
No amount of space budgeting and control can make a program
small. That requires invention and craftsmanship.
space Techniques 101
Obviously, more function means more space, speed being held
constant. So the first area of craftsmanship is in trading function
for size. Here there comes an early and deep policy question. How
much of that choice shall be reserved for the user? One can design
a program with many optional features, each of which takes a little
space. One can design a generator that will take an option Jist and
tailor a program to it. But for any particular set of options, a more
monolithic program would take less space. It's rather like a car; if
the map light, cigarette lighter, and clock are priced together as a
single option, the package will cost less than if one can choose each
separately. So the designer must decide how fine-grained the user
choice of options will be.
In designing a system for a range of memory sizes, another
basic question arises. A limiting effect keeps the range of suitabil-
ity from being made arbitrarily wide, even with fine-grained
modulaiity of function. In the smallest system, most modules will
be overlaid. A substantial part of the smallest system's resident
space must be set aside as a transient or paging area into which
other parts are fetched. The size of this determines the size of all
modules. And breaking functions into small modules costs both
performance and space. So a large system, which can afford a
transient area twenty times as large, only saves accesses thereby.
It still suffers in both speed and space because the module size is
so small. This effect limits the maximum efficient system that can
be generated from the modules of a small system.
The second area of craftsmanship is space-time trade-offs. For
a given function, the more space, the faster. This is true over an
amazingly large range. It is this fact that makes it feasible to set
The manager can do two things to help his team make good
space-time trade-offs. One is to ensure that they are trained in
programming technique, not just left to rely on native wit and
previous experience. For a new language or machine this is espe-
cially important. The peculiarities of its skillful use need to be
102 Ten Pounds in a Five-Pound Sack
learned quickly and shared widely, perhaps with special prizes or
praises for new techniques.
The second is to recognize that programming has a technol-
ogy, and components need to be fabricated. Every project needs
a notebook full of good subroutines or macros for queuing, search-
ing, hashing, and sorting. For each such function the notebook
should have at least two programs, the quick and the squeezed.
The development of such technology is an important realization
task that can be done in parallel with system architecture.
Representation Is the Essence of Programming
Beyond craftsmanship lies invention, and it is here that lean,
spare, fast programs are born. Almost always these are the result
of stategic breakthrough rather than tactical cleverness. Some-
times the strategic breakthrough will be a new algorithm, such as
the Cooley-Tukey Fast Fourier Transform or the substitution of
an n log n sort for an «^ set of comparisons.
Much more often, strategic breakthrough will come from
redoing the representation of the data or tables. This is where the
heart of a program lies. Show me your flowcharts and conceal your
tables, and I shall continue to be mystified. Show me your tables,
and I won't usually need your flowcharts; they'll be obvious.
It is easy to multiply examples of the power of representa-
tions. I recall a young man undertaking to build an elaborate
console interpreter for an IBM 650. He ended up packing it onto
an incredibly small amount of space by building an interpreter for
the interpreter, recognizing that human interactions are slow and
infrequent, but space was dear. Digitek's elegant little Fortran
compiler uses a very dense, specialized representation for the com-
piler code itself, so that external storage is not needed. That time
lost in decoding this representation is gained back tenfold by
avoiding input-output. (The exercises at the end of Chapter 6 in
Brooks and Iverson, Automatic Data Processing^ include a collection
of such examples, as do many of Knuth's exercises.^)
Representation Is the Essence of Programming 103
The programmer at wit's end for lack of space can often do
best by disentangling himself from his code, rearing back, and
contemplating his data. Representation is the essence of program-
Amid a wash of paper, a small number of documents
become the critical pivots around which every project 's
management revolves. These are the manager's chief
W. Bengough, "Scene in the old Congressional Library/' 1897
The Bettman Archive
108 The Documentary Hypothesis
The technology, the surrounding organization, and the traditions
of the craft conspire to define certain items of paperwork that a
project must prepare. To the new manager, fresh from operating
as a craftsman himself, these seem an unmitigated nuisance, an
unnecessary distraction, and a white tide that threatens to engulf
him. And indeed, most of them are exactly that.
Bit by bit, however, he comes to realize that a certain small set
of these documents embodies and expresses much of his manage-
rial work. The preparation of each one serves as a major occasion
for focusing thought and crystallizing discussions that otherwise
would wander endlessly. Its maintenance becomes his surveillance
and warning mechanism. The document itself serves as a check
list, a status control, and a data base for his reporting.
In order to see how this should work for a software project,
let us examine the specific documents useful in other contexts and
see if a generalization emerges.
Documents for a Computer Product
Suppose one is building a machine. What are the critical docu-
Objectives. This defines the need to be met and the goals,
desiderata, constraints, and priorities.
Specifications. This is a computer manual plus performance
specifications. It is one of the first documents generated in propos-
ing a new product, and the last document finished.
Budget. Not merely a constraint, the budget is one of the mana-
ger's most useful documents. Existence of the budget forces tech-
nical decisions that otherwise would be avoided; and, more
important, it forces and clarifies policy decisions.
Documents for a University Department 109
Estimate, forecast, prices. These three have cyclic interlocking,
which determines the success or failure of the project:
To generate a market forecast, one needs performance specifi-
cations and postulated prices. The quantities from the forecast
combine with component counts from the design to determine the
manufacturing cost estimate, and they determine the per-unit
share of development and fixed costs. These costs in turn deter-
If the prices are below those postulated, a joyous success spiral
begins. Forecasts rise, unit costs drop, and prices drop yet further.
If the prices are above those postulated, a disastrous spiral
begins, and all hands must struggle to break it. Performance must
be squeezed up and new applications developed to support larger
forecasts. Costs must be squeezed down to yield lower estimates.
The stress of this cycle is a discipline that often evokes the best
work of marketer and engineer.
It can also bring about ridiculous vacillation. I recall a machine
whose instruction counter popped in or out of memory every six
months during a three-year development cycle. At one phase a
little more performance would be wanted, so the instruction coun-
ter was implemented in transistors. At the next phase cost reduc-
tion was the theme, so the counter would be implemented as a
memory location. On another project the best engineering man-
ager I ever saw served often as a giant flywheel, his inertia damp-
ing the fluctuations that came from market and management
Documents for a University Department
In spite of the immense differences in purpose and activity, a
similar number of similar documents form the critical set for the
110 The Documentary Hypothesis
chairman of a university department. Almost every decision by
dean, faculty meeting, or chairman is a specification or change of
Research proposals (hence plans, when funded)
Class schedule and teaching assignments
Assignment of staff and graduate students
Notice that the components are very like those of the com-
puter project: objectives, product specifications, time allocations,
money allocations, space allocations, and people allocations. Only
the pricing documents are missing; here the legislature does that
task. The similarities are not accidental — the concerns of any man-
agement task are what, when, how much, where, and who.
Documents for a Software Project
In many software projects, people begin by holding meetings to
debate structure; then they start writing programs. No matter how
small the project, however, the manager is wise to begin immedi-
ately to formalize at least mini-documents to serve as his data
base. And he turns out to need documents much like those of other
What: objectives. This defines the need to be met and the goals,
desiderata, constraints, and priorities.
What: product specifications. This begins as a proposal and
ends up as the manual and internal documentation. Speed and
space specifications are a critical part.
Why Have Formal Documents? Ill
How much: budget
Where: space allocation
Who: organization chart. This becomes intertwined with the
interface specification, as Conway's Law predicts: ''Organizations
which design systems are constrained to produce systems which
are copies of the communication structures of these organiza-
tions/'^ Conway goes on to point out that the organization chart
will initially reflect the first system design, which is almost surely
not the right one. If the system design is to be free to change, the
organization must be prepared to change.
Why Have Formal Documents?
First, writing the decisions down is essential. Only when one
writes do the gaps appear and the inconsistencies protrude. The act
of writing turns out to require hundreds of mini-decisions, and it
is the existence of these that distinguishes clear, exact policies
from fuzzy ones.
Second, the documents will communicate the decisions to oth-
ers. The manager will be continually amazed that policies he took
for common knowledge are totally unknown by some member of
his team. Since his fundamental job is to keep everybody going in
the same direction, his chief daily task will be communication, not
decision-making, and his documents will immensely lighten this
Finally, a manager's documents give him a data base and
checklist. By reviewing them periodically he sees where he is, and
he sees what changes of emphasis or shifts in direction are needed.
1 do not share the salesman-projected vision of the "manage-
ment total-information system," wherein the executive strokes an
inquiry into a computer, and a display screen flashes his answer.
There are many fundamental reasons why this will never happen.
112 The Documentary Hypothesis
One reason is that only a small part — perhaps 20 percent — of the
executive's time is spent on tasks where he needs information
from outside his head. The rest is communication: hearing, report-
ing, teaching, exhorting, counseling, encouraging. But for the frac-
tion that is data-based, the handful of critical documents are vital,
and they will meet almost all needs.
The task of the manager is to develop a plan and then to realize
it. But only the written plan is precise and communicable. Such a
plan consists of documents on what, when, how much, where, and
who. This small set of critical documents encapsulates much of the
manager's work. If their comprehensive and critical nature is rec-
ognized in the beginning, the manager can approach them as
friendly tools rather than annoying busywork. He will set his
direction much more crisply and quickly by doing so.
There is nothing in this world constant but inconstancy,
It is common sense to take a method and try it. If it fails,
admit it frankly and try another. But above all, try
FRANKLIN D. ROOSEVELT^
Collapse of the aerodynamically misdesigned Tacoma Narrows Bridge,
116 Plan to Throw One Away
Pilot Plants and Scaling Up
Chemical engineers learned long ago that a process that works in
the laboratory cannot be implemented in a factory in only one
step. An intermediate step called the pilot plant is necessary to give
experience in scaling quantities up and in operating in nonprotec-
tive environments. For example, a laboratory process for desalting
water will be tested in a pilot plant of 10,000 gallon/day capacity
before being used for a 2,000,000 gallon/day community water
Programming system builders have also been exposed to this
lesson, but it seems to have not yet been learned. Project after
project designs a set of algorithms and then plunges into construc-
tion of customer-deliverable software on a schedule that demands
delivery of the first thing built.
In most projects, the first system built is barely usable. It may
be too slow, too big, awkward to use, or all three. There is no
alternative but to start again, smarting but smarter, and build a
redesigned version in which these problems are solved. The dis-
card and redesign may be done in one lump, or it may be done
piece-by-piece. But all large-system experience shows that it will
be done.^ Where a new system concept or new technology is used,
one has to build a system to throw away, for even the best plan-
ning is not so omniscient as to get it right the first time.
The management question, therefore, is not whether to build
a pilot system and throw it away. You will do that. The only
question is whether to plan in advance to build a throwaway, or
to promise to deliver the throwaway to customers. Seen this way,
the answer is much clearer. Delivering that throwaway to custom-
ers buys time, but it does so only at the cost of agony for the user,
distraction for the builders while they do the redesign, and a bad
reputation for the product that the best redesign will find hard to
Hence plan to throw one away; you will, anyhow.
Plan the System for Change 117
The Only Constancy Is Change Itself
Once one recognizes that a pilot system must be built and dis-
carded, and that a redesign with changed ideas is inevitable, it
becomes useful to face the whole phenomenon of change. The first
step is to accept the fact of change as a way of life, rather than an
untoward and annoying exception. Cosgrove has perceptively
pointed out that the programmer delivers satisfaction of a user
need rather than any tangible product. And both the actual need
and the user's perception of that need will change as programs are
built, tested, and used.^
Of course this is also true of the needs met by hardware
products, whether new cars or new computers. But the very exis-
tence of a tangible object serves to contain and quantize user
demand for changes. Both the tractability and the invisibility of
the software product expose its builders to perpetual changes in
Far be it from me to suggest that all changes in customer
objectives and requirements must, can, or should be incorporated
in the design. Clearly a threshold has to be established, and it must
get higher and higher as development proceeds, or no product ever
Nevertheless, some changes in objectives are inevitable, and it
is better to be prepared for them than to assume that they won't
come. Not only are changes in objective inevitable, changes in
development strategy and technique are also inevitable. The
throw-one-away concept is itself just an acceptance of the fact
that as one learns, he changes the design.^
Plan the System for Change
The ways of designing a system for each change are well known
and widely discussed in the literature — perhaps more widely dis-
118 Plan to Throw One Away
cussed than practiced. They include careful modularization, ex-
tensive subroutining, precise and complete definition of
intermodule interfaces, and complete documentation of these.
Less obviously one wants standard calling sequences and table-
driven techniques used wherever possible.
Most important is the use of a high-level language and self-
documenting techniques so as to reduce errors induced by
changes. Using compile-time operations to incorporate standard
declarations helps powerfully in making changes.
Quantization of change is an essential technique. Every prod-
uct should have numbered versions, and each version must have
its own schedule and a freeze date, after which changes go into the
Plan the Organization for Change
Cosgrove advocates treating all plans, milestones, and schedules as
tentative, so as to facilitate change. This goes much too far — the
common failing of programming groups today is too little manage-
ment control, not too much.
Nevertheless, he offers a great insight. He observes that the
reluctance to document designs is not due merely to laziness or
time pressure. Instead it comes from the designer's reluctance to
commit himself to the defense of decisions which he knows to be
tentative. ''By documenting a design, the designer exposes himself
to the criticisms of everyone, and he must be able to defend
everything he writes. If the organizational structure is threatening
in any way, nothing is going to be documented until it is com-
Structuring an organization for change is much harder than
designing a system for change. Each man must be assigned to jobs
that broaden him, so that the whole force is technically flexible.
On a large project the manager needs to keep two or three top
programmers as a technical cavalry that can gallop to the rescue
wherever the battle is thickest.
Plan the Organization for Change 119
Management structures also need to be changed as the system
changes. This means that the boss must give a great deal of atten-
tion to keeping his managers and his technical people as inter-
changeable as their talents allow.
The barriers are sociological, and they must be fought with
constant vigilance. First, managers themselves often think of se-
nior people as ''too valuable" to use for actual programming. Next,
management jobs carry higher prestige. To overcome this problem
some laboratories, such as Bell Labs, abolish all job titles. Each
professional employee is a ''member of the technical staff." Oth-
ers, like IBM, maintain a dual ladder of advancement, as Fig. 11.1
shows. The corresponding rungs are in theory equivalent.
Managerial Ladder Technical Ladder
Senior Programmer Senior Programmer
Development Programmer Advisory Programmer
Project Programmer Staff Programmer
Senior Associate Programmer
Fig. 11.1 IBM dual ladder of advancement
It is easy to establish corresponding salary scales for rungs. It
is much harder to give them corresponding prestige. Offices have
to be of equal size and appointment. Secretarial and other support
services must correspond. A reassignment from the technical lad-
der to a corresponding level on the managerial one should never
be accompanied by a raise, and it should be announced always as
120 Plan to Throw One Away
a ''reassignment/' never as a "promotion/' The reverse reassign-
ment should always carry a raise; overcompensating for the cul-
tural forces is necessary.
Managers need to be sent to technical refresher courses, senior
technical people to management training. Project objectives,
progress, and management problems must be shared with the
whole body of senior people.
Whenever talents permit, senior people must be kept techni-
cally and emotionally ready to manage groups or to delight in
building programs with their own hands. Doing this surely is a lot
of work; but it surely is worth it!
The whole notion of organizing surgical-type programming
teams is a radical attack on this problem. It has the effect of making
a senior man feel that he does not demean himself when he builds
programs, and it attempts to remove the social obstacles that de-
prive him of that creative joy.
Furthermore, that structure is designed to minimize the num-
ber of interfaces. As such, it makes the system maximally easy to
change, and it becomes relatively easy to reassign a whole surgical
team to a different programming task when organizational changes
are necessary. It is really the long-run answer to the problem of
Two Steps Forward and One Step Back
A program doesn't stop changing when it is delivered for customer
use. The changes after delivery are called program maintenance, but
the process is fundamentally different from hardware mainte-
Hardware maintenance for a computer system involves three
activities — replacing deteriorated components, cleaning and lu-
bricating, and putting in engineering changes that fix design de-
fects. (Most, but not all, engineering changes fix defects in the
realization or implementation, rather than the architecture, and so
are invisible to the user.)
Two Steps Forward and One Step Back 121
Program maintenance involves no cleaning, lubrication, or re-
pair of deterioration. It consists chiefly of changes that repair
design defects. Much more often than with hardware, these
changes include added functions. Usually they are visible to the
The total cost of maintaining a widely used program is typi-
cally 40 percent or more of the cost of developing it. Surprisingly,
this cost is strongly affected by the number of users. More users
find more bugs.
Betty Campbell, of MIT's Laboratory for Nuclear Science,
points out an interesting cycle in the life of a particular release of
a program. It is shown in Fig. 11.2. Initially, old bugs found and
solved in previous releases tend to reappear in a new release. New
functions of the new release turn out to have defects. These things
get shaken out, and all goes well for several months. Then the bug
rate begins to climb again. Miss Campbell believes this is due to
the arrival of users at a new plateau of sophistication, where they
begin to exercise fully the new capabilities of the release. This
intense workout then smokes out the more subtle bugs in the new
Months since installation
Fig. 11.2 Bug occurrence as a function of release age
122 Plan to Throw One Away
The fundamental problem with program maintenance is that
fixing a defect has a substantial (20-50 percent) chance of intro-
ducing another. So the whole process is two steps forward and one
Why aren't defects fixed more cleanly? First, even a subtle
defect shows itself as a local failure of some kind. In fact it often
has system-wide ramifications, usually nonobvious. Any attempt
to ^x it with minimum effort will repair the local and obvious, but
unless the structure is pure or the documentation very fine, the
far-reaching effects of the repair will be overlooked. Second, the
repairer is usually not the man who wrote the code, and often he
is a junior programmer or trainee.
As a consequence of the introduction of new bugs, program
maintenance requires far more system testing per statement writ-
ten than any other programming. Theoretically, after each fix one
must run the entire bank of test cases previously run against the
system, to ensure that it has not been damaged in an obscure way.
In practice such regression testing must indeed approximate this
theoretical ideal, and it is very costly.
Clearly, methods of designing programs so as to eliminate or
at least illuminate side effects can have an immense payoff in
maintenance costs. So can methods of implementing designs with
fewer people, fewer interfaces, and hence fewer bugs.
One Step Forward and One Step Back
Lehman and Belady have studied the history of successive releases
in a large operating system.^ They find that the total number of
modules increases linearly with release number, but that the num-
ber of modules affected increases exponentially with release num-
ber. All repairs tend to destroy the structure, to increase the
entropy and disorder of the system. Less and less effort is spent
on fixing original design flaws; more and more is spent on fixing
flaws introduced by earlier fixes. As time passes, the system
becomes less and less well-ordered. Sooner or later the fixing
One Step Forward and One Step Back 123
ceases to gain any ground. Each forward step is matched by a
backward one. Although in principle usable forever, the system
has worn out as a base for progress. Furthermore, machines
change, configurations change, and user requirements change, so
the system is not in fact usable forever. A brand-new, from-the-
ground-up redesign is necessary.
And so from a statistical mechanical model, Belady and Leh-
man arrive for programming-systems at a more general conclusion
supported by the experience of all the earth. 'Things are always
at their best in the beginning,'' said Pascal. C. S. Lewis has stated
it more perceptively:
That is the key to history. Terrific energy is expended — civilizations
are built up — excellent institutions devised; but each time something
goes wrong. Some fatal flaw always brings the selfish and cruel people
to the top, and then it all slides back into misery and ruin. In fact,
the machine conks. It seems to start up all right and runs a few yards,
and then it breaks down. ^
Systems program building is an entropy-decreasing process,
hence inherently metastable. Program maintenance is an entropy-
increasing process, and even its most skillful execution only delays
the subsidence of the system into unfixable obsolescence.
A good workman is known by his tools.
A. Pisano, "Lo Scultore/' from the Campanile di Santa Maria del
Fiore, Florence, c. 1335
Scala, New York/Pirenze and Foto Alinari, Firenze
128 Sharp Tools
Even at this late date, many programming projects are still oper-
ated like machine shops so far as tools are concerned. Each master
mechanic has his own personal set, collected over a lifetime and
carefully locked and guarded — the visible evidences of personal
skills. Just so, the programmer keeps little editors, sorts, binary
dumps, disk space utilities, etc., stashed away in his file.
Such an approach, however, is foolish for a programming
project. First, the essential problem is communication, and indi-
vidualized tools hamper rather than aid communication. Second,
the technology changes when one changes machines or working
language, so tool lifetime is short. Finally, it is obviously much
more efficient to have common development and maintenance of
the general-purpose programming tools.
General-purpose tools are not enough, however. Both special-
ized needs and personal preferences dictate the need for special-
ized tools as well; so in discussing programming teams I have
postulated one toolmaker per team. This man masters all the com-
mon tools and is able to instruct his client-boss in their use. He
also builds the specialized tools his boss needs.
The manager of a project, then, needs to establish a philoso-
phy and set aside resources for the building of common tools. At
the same time he must recognize the need for specialized tools, and
not begrudge his working teams their own tool-building. This
temptation is insidious. One feels that if all those scattered tool
builders were gathered in to augment the common tool team,
greater efficiency would result. But it is not so.
What are the tools about which the manager must philoso-
phize, plan, and organize? First, a computer faciliti/. This requires
machines, and a scheduling philosophy must be adopted. It re-
quires an operating system, and service philosophies must be estab-
lished. It requires language, and a language policy must be laid
down. Then there are utilities, debugging aids, test-case generators,
and a text-processing system to handle documentation. Let us look
at these one by one.^
Target Machines 129
Machine support is usefully divided into the target machine and the
vehicle machines. The target machine is the one for which software
is being written, and on which it must ultimately be tested. The
vehicle machines are those that provide the services used in build-
ing the system. If one is building a new operating system for an
old machine, it may serve not only as the target, but as the vehicle
What kind of target facility? Teams building new supervisors or
other system-heart software will of course need machines of their
own. Such systems will need operators and a system programmer
or two who keeps the standard support on the machine current
If a separate machine is needed, it is a rather peculiar thing —
it need not be fast, but it needs at least a million bytes of main
storage, a hundred million bytes of on-line disk, and terminals.
Only alphanumeric terminals are needed, but they must go much
faster than the 15 characters per second that characterizes type-
writers. A large memory adds greatly to productivity by allowing
overlaying and size trimming to be done after functional testing.
The debugging machine, or its software, also needs to be in-
strumented, so that counts and measurements of all kinds of pro-
gram parameters can be automatically made during debugging.
Memory-use patterns, for instance, are powerful diagnostics of
the causes of weird logical behavior or unexpectedly slow perfor-
Scheduling. When the target machine is new, as when its first
operating system is being built, machine time is scarce, and sched-
uling it is a major problem. The requirement for target machine
time has a pecuHar growth curve. In OS/360 development we had
good System/360 simulators and other vehicles. From previous
experience we projected how many hours of S/360 time we would
need, and began to acquire early machines from factory produc-
130 Sharp Tools
tion. But they sat idle, month after month. Then all at once all 16
systems were fully loaded, and rationing was the problem. The
utilization looked something like Fig. 12.1. Everyone began to
debug his first components at the same time, and thereafter most
of the team was constantly debugging something.
Model 40 hours
Jan '65 '66
Fig. 12.1 Growth in use of target machines
We centralized all our machines and tape library and set up a
professional and experienced machine-room team to run them. To
maximize scarce S/360 time, we ran all debugging runs in batch
on whichever system was free and appropriate. We tried for four
shots per day (two-and-one-half-hour turnaround) and de-
manded four-hour turnaround. An auxiliary 1401 with terminals
was used to schedule runs, to keep track of the thousands of jobs,
and to monitor turnaround time.
But all that organization was quite overdone. After a few
months of slow turnaround, mutual recriminations, and other
agony, we went to allocating machine time in substantial blocks.
Vehicle Machines and Data Services 131
The whole fifteen-man sort team, for example, would be given a
system for a four-to-six-hour block. It was up to them to schedule
themselves on it. If it sat idle, no outsider could use it.
That, it develops, was a better way to allocate and schedule.
Although machine utilization may have been a little lower (and
often it wasn't), productivity was way up. For each man on such
a team, ten shots in a six-hour block are far more productive than
ten shots spaced three hours apart, because sustained concentra-
tion reduces thinking time. After such a sprint, a team usually
needed a day or two to catch up on the paperwork before asking
for another block. Often as few as three programmers can fruit-
fully share and subschedule a block of time. This seems to be the
best way to use a target machine when debugging a new operating
It has always been so in practice, though never in theory.
System debugging has always been a graveyard-shift occupation,
like astronomy. Twenty years ago, on the 701, 1 was initiated into
the productive informality of the predawn hours, when all the
machine-room bosses are fast asleep at home, and the operators
are disinclined to be sticklers for rules. Three machine generations
have passed; technologies have changed totally; operating systems
have arisen; and yet this preferred method of working hasn't
changed. It endures because it is most productive. The time has
come to recognize its productivity and embrace the fruitful prac-
Vehicle Machines and Data Services
Simulators. If the target computer is new, one needs a logical
simulator for it. This gives a debugging vehicle long before the real
target exists. Equally important, it gives access to a dependable
debugging vehicle even after one has a target machine available.
Dependable is not the same as accurate. The simulator will
surely fail in some respect to be a faithful and accurate implemen-
132 Sharp Tools
tation of the new machine's architecture. But it will be the same
innplementation from one day to the next, and the new hardware
We are accustomed nowadays to having computer hardware
work correctly almost all the time. Unless an application program-
mer sees a system behaving inconsistently from run to identical
run, he is well advised to look for bugs in his code rather than in
This experience, however, is bad training for the programming
of support for a new machine. Lab-built, preproduction, or early
hardware does not work as defined, does not work reliably, and
does not stay the same from day to day. As bugs are found,
engineering changes are made in all machine copies, including
those of the programming group. This shifting base is bad enough.
Hardware failures, usually intermittent, are worse. The uncer-
tainty is worst of all, for it robs one of incentive to dig diligently
in his code for a bug — it may not be there at all. So a dependable
simulator on a well-aged vehicle retains its usefulness far longer
than one would expect.
Compiler and assembler vehicles. For the same reasons, one
wants compilers and assemblers that run on dependable vehicles
but compile object code for the target system. This can then start
being debugged on the simulator.
With high-level language programming, one can do much of
the debugging by compiling for and testing object code on the
vehicle machine before beginning to test target-machine code at
all. This gives the efficiency of direct execution, rather than that
of simulation, combined with the dependability of the stable ma-
Program libraries and accounting. A very successful and im-
portant use of a vehicle machine in the OS/360 development effort
was for the maintenance of program libraries. A system developed
under the leadership of W. R. Crowley had two 7010's connected,
sharing a large disk data bank. The 7010's also provided an S/360
Vehicle Machines and Data Services 133
assembler. All the code tested or under test was kept in this li-
brary, both source code and assembled load modules. The library
was in fact divided into sublibraries with different access rules.
First, each group or programmer had an area where he kept
copies of his programs, his test cases, and the scaffolding he
needed for component testing. In this playpen area there were no
restrictions on what a man could do with his own programs; they
When a man had his component ready for integration into a
larger piece, he passed a copy over to the manager of that larger
system, who put this copy into a system integration sub library. Now
the original programmer could not change it, except by permission
of the integration manager. As the system came together, the latter
would proceed with all sorts of system tests, identifying bugs and
From time to time a system version would be ready for wider
use. Then it would be promoted to the current version sublibrary.
This copy was sacrosanct, touched only to fix crippling bugs. It
was available for use in integration and testing of all new module
versions. A program directory on the 7010 kept track of each
version of each module, its status, its whereabouts, and its
Two notions are important here. The first is control, the idea
of program copies belonging to managers who alone can authorize
their change. The second is that of formal separation and progression
from the playpen, to integration, to release.
In my opinion this was one of the best-done things in the
OS/360 effort. It is a piece of management technology that seems
to have been independently developed on several massive pro-
gramming projects including those at Bell Labs, ICL, and Cam-
bridge University.^ It is applicable to documentation as well as to
programs. It is an indispensable technology.
Program tools. As new debugging techniques appear, the old
ones diminish but do not vanish. Thus one needs dumps, source-
file editors, snapshot dumps, even traces.
134 Sharp Tools
Likewise one needs a full set of utilities for putting decks on
disks, making tape copies, printing files, changing catalogs. If one
commissions a project toolmaker early in the process, these can be
done once and can be ready by time they are needed.
Documentation system. Among all tools, the one that saves the
most labor may well be a computerized text-editing system, oper-
ating on a dependable vehicle. We had a very handy one, devised
by J. W. Franklin. Without it 1 expect OS/360 manuals would have
been far later and more cryptic. There are those who would argue
that the OS/360 six-foot shelf of manuals represents verbal diarr-
hea, that the very voluminosity introduces a new kind of incom-
prehensibility. And there is some truth in that.
But I respond in two ways. First, the OS/360 documentation
is overwhelming in bulk, but the reading plan is carefully laid out;
if one uses it selectively, he can ignore most of the bulk most of
the time. One must consider the OS/360 documentation as a li-
brary or an encyclopedia, not a set of mandatory texts.
Second, this is far preferable to the severe underdocumenta-
tion that characterizes most programming systems. 1 will quickly
agree, however, that the writing could be vastly improved in some
places, and that the result of better writing would be reduced bulk.
Some parts (e.g.. Concepts and Facilities) are very well-written now.
Performance simulator. Better have one. Build it outside-in, as
we will discuss in the next chapter. Use the same top-down design
for the performance simulator, the logical simulator, and the prod-
uct. Start it very early. Listen to it when it speaks.
High-Level Language and Interactive Programming
The most important two tools for system programming today are
two that were not used in OS/360 development almost a decade
ago. They are still not widely used, but all evidence points to their
power and applicability. They are (1) high-level language and
(2) interactive programming. I am convinced that only inertia and
High-Level Language and Interactive Programming 135
sloth prevent the universal adoption of these tools; the technical
difficulties are no longer valid excuses.
High-level language. The chief reasons for using a high-level
language are productivity and debugging speed. We have dis-
cussed productivity earlier (Chapter 8). There is not a lot of nu-
merical evidence, but what there is suggests improvement by
integral factors, not just incremental percentages.
The debugging improvement comes from the fact that there
are fewer bugs, and they are easier to find. There are fewer because
one avoids an entire level of exposure to error, a level on which
one makes not only syntactic errors but semantic ones, such as
misusing registers. The bugs are easier to find because the compiler
diagnostics help find them and, more important, because it is very
easy to insert debugging snapshots.
For me, these productivity and debugging reasons are over-
whelming. 1 cannot easily conceive of a programming system I
would build in assembly language.
Well, what about the classical objections to such a tool? There
are three: It doesn't let me do what I want. The object code is too
big. The object code is too slow.
As to function, I believe the objection is no longer valid. All
testimony indicates that one can do what he needs to do, but that
it takes work to find out how, and one may occasionally need
As to space, the new optimizing compilers are beginning to be
very satisfactory, and this improvement will continue.
As to speed, optimizing compilers now produce some code
that is faster than most programmer's handwritten code, Further-
more, one can usually solve speed problems by replacing from one
to five percent of a compiler-generated program by handwritten
substitute after the former is fully debugged.^
What high-level language should one use for system program-
ming? The only reasonable candidate today is PL/I.® It has a very
136 Sharp Tools
full set of functions; it is matched to operating system environ-
ments; and a variety of compilers are available, some interactive,
some fast, some very diagnostic, and some producing highly opti-
mized code. I myself find it faster to work out algorithms in APL;
then 1 translate these to PL/I for matching to the system environ-
Interactive programming. One of the justifications for MIT's
Multics project was its usefulness for building programming sys-
tems. Multics (and following it, IBM's TSS) differs in concept from
other interactive computing systems in exactly those respects nec-
essary for systems programming: many levels of sharing and pro-
tection for data and programs, extensive library management, and
facilities for cooperative work among terminal users. I am con-
vinced that interactive systems will never displace batch systems
for many applications. But I think the Multics team has made its
most convincing case in the system-programming application.
There is not yet much evidence available on the true fruitful-
ness of such apparently powerful tools. There is a widespread
recognition that debugging is the hard and slow part of system
programming, and slow turnaround is the bane of debugging. So
the logic of interactive programming seems inexorable.^
Batch (B) or
7094 ESS support
2100-3400 ^..„, ,
360 ESS support
360 ESS support
Fig. 12.2 Comparative productivity under batch and conversational pro-
High-Level Language and Interactive Programming 137
Further, we hear good testimonies from many who have built
little systems or parts of systems in this way. The only numbers
I have seen for effects on programming of large systems were
reported by John Harr of Bell Labs. They are shown in Fig. 12.2.
These numbers are for writing, assembling, and debugging pro-
grams. The first program is mostly control program; the other three
are language translators, editors, and such. Harr's data suggest that
an interactive facihty at least doubles productivity in system pro-
The effective use of most interactive tools requires that the
work be done in a high-level language, for teletype and typewriter
terminals cannot be used to debug by dumping memory. With a
high-level language, source can be easily edited and selective
printouts easily done. Together they make a pair of sharp tools
The Whole and the Parts
The Whole and the Parts
I can call spirits from the vasty deep.
Why so can 1, or so can any man; hut will they come
when you do call for them?
SHAKESPEARE, KING HENRY IV, PART I
© Walt Disney Productions
142 The Whole and the Parts
The modern magic, like the old, has its boastful practitioners: 'T
can write programs that control air traffic, intercept ballistic mis-
siles, reconcile bank accounts, control production lines." To which
the answer comes, "So can I, and so can any man, but do they work
when you do write them?"
How does one build a program to work? How does one test
a program? And how does one integrate a tested set of component
programs into a tested and dependable system? We have touched
upon the techniques here and there; let us now consider them
somewhat more systematically.
Designing the Bugs Out
Bug-proofing the definition. The most pernicious and subtle
bugs are system bugs arising from mismatched assumptions made
by the authors of various components. The approach to conceptual
integrity discussed above in Chapters 4, 5, and 6 addresses these
problems directly. In short, conceptual integrity of the product not
only makes it easier to use, it also makes it easier to build and less
subject to bugs.
So does the detailed, painstaking architectural effort implied
by that approach. V. A. Vyssotsky, of Bell Telephone Laborato-
ries' Safeguard Project, says, "The crucial task is to get the product
defined. Many, many failures concern exactly those aspects that
were never quite specified."^ Careful function definition, careful
specification, and the disciplined exorcism of frills of function and
flights of technique all reduce the number of system bugs that
have to be found.
Testing the specification. Long before any code exists, the spec-
ification must be handed to an outside testing group to be scruti-
nized for completeness and clarity. As Vyssotsky says, the
developers themselves cannot do this: "They won't tell you they
don't understand it; they will happily invent their way through
the gaps and obscurities."
Designing the Bugs Out 143
Top-down design. In a very clear 1971 paper, Niklaus Wirth
formalized a design procedure which had been used for years by
the best programmers.^ Furthermore, his notions, although stated
for program design, apply completely to the design of complex
systems of programs. The division of system building into archi-
tecture, implementation, and realization is an embodiment of
these notions; furthermore, each of the architecture, implementa-
tion, and realization can be best done by top-down methods.
Briefly, Wirth's procedure is to identify design as a sequence
of refinement steps. One sketches a rough task definition and a
rough solution method that achieves the principal result. Then one
examines the definition more closely to see how the result differs
from what is wanted, and one takes the large steps of the solution
and breaks them down into smaller steps. Each refinement in the
definition of the task becomes a refinement in the algorithm for
solution, and each may be accompanied by a refinement in the
From this process one identifies modules of solution or of data
whose further refinement can proceed independently of other
work. The degree of this modularity determines the adaptability
and changeability of the program.
Wirth advocates using as high-level a notation as is possible
at each step, exposing the concepts and concealing the details until
further refinement becomes necessary.
A good top-down design avoids bugs in several ways. First,
the clarity of structure and representation makes the precise state-
ment of requirements and functions of the modules easier. Second,
the partitioning and independence of modules avoids system bugs.
Third, the suppression of detail makes flaws in the structure more
apparent. Fourth, the design can be tested at each of its refinement
steps, so testing can start earlier and focus on the proper level of
detail at each step.
The process of step-wise refinement does not mean that one
never has to go back, scrap the top level, and start the whole thing
144 The Whole and the Parts
again as he encounters some unexpectedly knotty detail. Indeed,
that happens often. But it is much easier to see exactly when and
why one should throw away a gross design and start over. Many
poor systems come from an attempt to salvage a bad basic design
and patch it with all kinds of cosmetic relief. Top-down design
reduces the temptation.
I am persuaded that top-down design is the most important
new programming formalization of the decade.
Structured programming. Another important set of new ideas
for designing the bugs out of programs derives largely from
Dijkstra,^ and is built on a theoretical structure by Bohm and
Basically the approach is to design programs whose control
structures consist only of loops defined by a statement such as DO
WHILE, and conditional portions delineated into groups of state-
ments marked with brackets and conditioned by an IF . . . THEN
. . . ELSE. Bohm and Jacopini show these structures to be theoreti-
cally sufficient; Dijkstra argues that the alternative, unrestrained
branching via GO TO, produces structures that lend themselves
to logical errors.
The basic notion is surely sound. Many criticisms have been
made, and additional control structures, such as an n-way branch
(the so-called CASE statement) for distinguishing among many
contingencies, and a disaster bail-out (GO TO ABNORMAL
END) are very convenient. Further, some have become very doc-
trinaire about avoiding all GO TO's, and that seems excessive.
The important point, and the one vital to constructing bug-
free programs, is that one wants to think about the control struc-
tures of a system as control structures, not as individual branch
statements. This way of thinking is a major step forward.
The procedures for debugging programs have been through a great
cycle in the past twenty years, and in some ways they are back
Component Debugging 145
where they started. The cycle has gone through four steps, and it
is fun to trace them and see the motivation for each.
On-machine debugging. Early machines had relatively poor in-
put-output equipment, and long input-output delays. Typically,
the machine read and wrote paper tape or magnetic tape and
off-line facilities were used for tape preparation and printing. This
made tape input-output intolerably awkward for debugging, so
the console was used instead. Thus debugging was designed to
allow as many trials as possible per machine session.
The programmer carefully designed his debugging procedure
— planning where to stop, what memory locations to examine,
what do find there, and what to do if he didn't. This meticulous
programming of himself as a debugging machine might well take
half as long as writing the computer program to be debugged.
The cardinal sin was to push START boldly without having
segmented the program into test sections with planned stops.
Memory dumps. On-machine debugging was very effective. In
a two-hour session, one could get perhaps a dozen shots. But
computers were very scarce, and very costly, and the thought of
all that machine time going to waste was horrifying.
So when high-speed printers were attached on-line, the tech-
nique changed. One ran a program until a check failed, and then
dumped the whole memory. Then began the laborious desk work,
accounting for each memory location's contents. The desk time
was not much different than that for on-machine debugging; but
it occurred after the test run, in deciphering, rather than before,
in planning. Debugging for any particular user took much longer,
because test shots depended upon batch turnaround time. The
whole procedure, however, was designed to minimize computer
time use, and to serve as many programmers as possible.
Snapshots. The machines on which memory dumping was de-
veloped had 2000-4000 words, or 8K to 16K bytes of memory. But
memory sizes grew by leaps and bounds, and total memory dump-
ing became impractical. So people developed techniques for selec-
146 The Whole and the Parts
tive dumping, selective tracing, and for inserting snapshots into
programs. The OS/360 TESTRAN is an end-of-the-line in this
direction, allowing one to insert snapshots into a program without
reassembly or recompilation.
Interactive debugging. In 1959 Codd and his coworkers^ and
Strachey® each reported work aimed at time-shared debugging, a
way of achieving both the instant turnaround of on-machine
debugging and the efficient machine use of batch debugging. The
computer would have multiple programs in memory, ready for
execution. A terminal, controlled only by program, would be asso-
ciated with each program being debugged. Debugging would be
under control of a supervisory program. When the programmer at
a terminal stopped his program to examine progress or to make
changes, the supervisor would run another program, thus keeping
the machines busy.
Codd's multiprogramming system was developed, but the em-
phasis was on throughput enhancement by efficient input-output
utilization, and interactive debugging was not implemented. Stra-
chey's ideas were improved and implemented in 1963 in an experi-
mental system for the 7090 by Corbato and colleagues at MIT.^
This development led to the MULTICS, TSS, and other time-
sharing systems of today.
The chief user-perceived differences between on-machine
debugging as first practiced and the interactive debugging of today
are the facilities made possible by the presence of the supervisory
program and its associated language interpreters. One can program
and debug in a high-level language. Efficient editing facilities
make changes and snapshots easy.
Return to the instant-turnaround capability of on-machine
debugging has not yet brought a return to the preplanning of
debugging sessions. In a sense such preplanning is not so necessary
as before, since machine time doesn't waste away while one sits
Nevertheless, Gold's interesting experimental results show
that three times as much progress in interactive debugging is made
on the first interaction of each session as on subsequent interac-
System Debugging 147
tions.® This strongly suggests that we are not realizing the poten-
tial of interaction due to lack of session planning. The time has
come to dust off the old on-machine techniques.
I find that proper use of a good terminal system requires two
hours at the desk for each two-hour session on the terminal. Half
of this time is spent in sweeping up after the last session: updating
my debugging log, filing updated program listings in my system
notebook, explaining strange phenomena. The other half is spent
in preparation: planning changes and improvements and designing
detailed tests for next time. Without such planning, it is hard to
stay productive for as much as two hours. Without the post-
session sweep-up, it is hard to keep the succession of terminal
sessions systematic and forward-moving.
Test cases. As for the design of actual debugging procedures and
test cases, Gruenberger has an especially good treatment,^ and
there are shorter treatments in other standard texts. ^°'^^
The unexpectedly hard part of building a programming system is
system test. 1 have already discussed some of the reasons for both
the difficulty and its unexpectedness. From all of that, one should
be convinced of two things: system debugging will take longer
than one expects, and its difficulty justifies a thoroughly system-
atic and planned approach. Let us now see what such an approach
Use debugged components. Common sense, if not common
practice, dictates that one should begin system debugging only
after the pieces seem to work.
Common practice departs from this in two ways. First is the
bolt-it-together-and-try approach. This seems to be based on the
notion that there will be system (i.e., interface) bugs in addition
to the component bugs. The sooner one puts the pieces together,
the sooner the system bugs will emerge. Somewhat less sophis-
ticated is the notion that by using the pieces to test each other, one
148 The Whole and the Parts
avoids a lot of test scaffolding. Both of these are obviously true,
but experience shows that they are not the whole truth — the use
of clean, debugged components saves much more time in system
testing than that spent on scaffolding and thorough component
A little more subtle is the ''documented bug" approach. This
says that a component is ready to enter system test when all the
flaws are found, well before the time when all are fixed. Then in
system testing, so the theory goes, one knows the expected effects
of these bugs and can ignore those effects, concentrating on the
All this is just wishful thinking, invented to rationalize away
the pain of slipped schedules. One does not know all the expected
effects of known bugs. If things were straightforward, system
testing wouldn't be hard. Furthermore, the fixing of the docu-
mented component bugs will surely inject unknown bugs, and
then system test is confused.
Build plenty of scaffolding. By scaffolding I mean all programs
and data built for debugging purposes but never intended to be in
the final product. It is not unreasonable for there to be half as
much code in scaffolding as there is in product.
One form of scaffolding is the dummy component, which con-
sists only of interfaces and perhaps some faked data or some small
test cases. For example, a system may include a sort program
which isn't finished yet. Its neighbors can be tested by using a
dummy program that merely reads and tests the format of input
data, and spews out a set of well-formatted meaningless but or-
Another form is the miniature file. A very common form of
system bug is misunderstanding of formats for tape and disk files.
So it is worthwhile to build some little files that have only a few
typical records, but all the descriptions, pointers, etc.
The limiting case of miniature file is the dummy file, which
really isn't there at all. OS/360's Job Control Language provides
such facility, and it is extremely useful for component debugging.
System Debugging 149
Yet another form of scaffolding are auxiliary programs. Gener-
ators for test data, special analysis printouts, cross-reference table
analyzers, are all examples of the special-purpose jigs and fixtures
one may want to build/^
Control changes. Tight control during test is one of the impres-
sive techniques of hardware debugging, and it applies as well to
First, somebody must be in charge. He and he alone must
authorize component changes or substitution of one version for
Then, as discussed above, there must be controlled copies of
the system: one locked-up copy of the latest versions, used for
component testing; one copy under test, with fixes being installed;
playpen copies where each man can work away on his component,
doing both fixes and extensions.
In System/360 engineering models, one saw occasional
strands of purple wire among the routine yellow wires. When a
bug was found, two things were done. A quick fix was devised and
installed on the system, so testing could proceed. This change was
put on on purple wire, so it stuck out like a sore thumb. It was
entered in the log. Meanwhile, an official change document was
prepared and started into the design automation mill. Eventually
this resulted in updated drawings and wire lists, and a new back
panel in which the change was implemented in printed circuitry
or yellow wire. Now the physical model and the paper were to-
gether again, and the purple wire was gone.
Programming needs a purple-wire technique, and it badly
needs tight control and deep respect for the paper that ultimately
is the product. The vital ingredients of such technique are the
logging of all changes in a journal and the distinction, carried
conspicuously in source code, between quick patches and
thought-through, tested, documented fixes.
Add one component at a time. This precept, too, is obvious, but
optimism and laziness tempt us to violate it. To do it requires
150 The Whole and the Parts
dummies and other scaffolding, and that takes work. And after all,
perhaps all that work won't be needed? Perhaps there are no bugs?
No! Resist the temptation! That is what systematic system
testing is all about. One must assume that there will be lots of
bugs, and plan an orderly procedure for snaking them out.
Note that one must have thorough test cases, testing the par-
tial systems after each new piece is added. And the old ones, run
successfully on the last partial sum, must be rerun on the new one
to test for system regression.
Quantize updates. As the system comes up, the component
builders will from time to time appear, bearing hot new versions
of their pieces — faster,smaller, more complete, or putatively less
buggy. The replacement of a working component by a new version
requires the same systematic testing procedure that adding a new
component does, although it should require less time, for more
complete and efficient test cases will usually be available.
Each team building another component has been using the
most recent tested version of the integrated system as a test bed
for debugging its piece. Their work will be set back by having that
test bed change under them. Of course it must. But the changes
need to be quantized. Then each user has periods of productive
stability, interrupted by bursts of test-bed change. This seems to
be much less disruptive than a constant rippling and trembling.
Lehman and Belady offer evidence that quanta should be very
large and widely spaced or else very small and frequent.^'* The
latter strategy is more subject to instability, according to their
model. My experience confirms it: I would never risk that strategy
Quantized changes neatly accommodate a purple-wire tech-
nique. The quick patch holds until the next regular release of the
component, which should incorporate the fix in tested and docu-
Hatching a Catastrophe
Hatching a Catastrophe
None love the bearer of bad news.
How does a project get to be a year late?
. . , One day at a time.
A. Canova, "Ercole e Lica/' 1802. Hercules hurls to his death the
messenger Lycas, who innocently brought the death-garment.
Scala, New York/Firenze and Foto Alinari, Firenze
154 Hatching a Catastrophe
When one hears of disastrous schedule sHppage in a project, he
imagines that a series of major calamities must have befallen it.
Usually, however, the disaster is due to termites, not tornadoes;
and the schedule has slipped imperceptibly but inexorably. In-
deed, major calamities are easier to handle; one responds with
major force, radical reorganization, the invention of new ap-
proaches. The whole team rises to the occasion.
But the day-by-day slippage is harder to recognize, harder to
prevent, harder to make up. Yesterday a key man was sick, and a
meeting couldn't be held. Today the machines are all down, be-
cause lightning struck the building's power transformer. Tomor-
row the disk routines won't start testing, because the first disk is
a week late from the factory. Snow, jury duty, family problems,
emergency meetings with customers, executive audits — the list
goes on and on. Each one only postpones some activity by a
half-day or a day. And the schedule slips, one day at a time.
Milestones or Millstones?
How does one control a big project on a tight schedule? The first
step is to have a schedule. Each of a list of events, called milestones,
has a date. Picking the dates is an estimating problem, discussed
already and crucially dependent on experience.
For picking the milestones there is only one relevant rule.
Milestones must be concrete, specific, measurable events, defined
with knife-edge sharpness. Coding, for a counterexample, is ''90
percent finished" for half of the total coding time. Debugging is
"99 percent complete" most of the time. "Planning complete" is
an event one can proclaim almost at will.^
Concrete milestones, on the other hand, are 100-percent
events. "Specifications signed by architects and implementers,"
"source coding 100 percent complete, keypunched, entered into
disk library," "debugged version passes all test cases." These con-
crete milestones demark the vague phases of planning, coding,
The Other Piece Is Late, Anyway" 155
It is more important that milestones be sharp-edged and un-
ambiguous than that they be easily verifiable by the boss. Rarely
will a man lie about milestone progress, if the milestone is so sharp
that he can't deceive himself. But if the milestone is fuzzy, the boss
often understands a different report from that which the man
gives. To supplement Sophocles, no one enjoys bearing bad news,
either, so it gets softened without any real intent to deceive.
Two interesting studies of estimating behavior by government
contractors on large-scale development projects show that:
1. Estimates of the length of an activity, made and revised care-
fully every two weeks before the activity starts, do not signifi-
cantly change as the start time draws near, no matter how
wrong they ultimately turn out to be.
2. During the activity, over esimxaies of duration come steadily
down as the activity proceeds.
3. Underestimates do not change significantly during the activity
until about three weeks before the scheduled completion.^
Sharp milestones are in fact a service to the team, and one they
can properly expect from a manager. The fuzzy milestone is the
harder burden to live with. It is in fact a millstone that grinds
down morale, for it deceives one about lost time until it is ir-
remediable. And chronic schedule slippage is a morale-killer.
"The Other Piece Is Late, Anyway"
A schedule slips a day; so what? Who gets excited about a one-day
slip? We can make it up later. And the other piece into which ours
fits is late, anyway.
A baseball manager recognizes a nonphysical talent, hustle, as
an essential gift of great players and great teams. It is the charac-
teristic of running faster than necessary, moving sooner than nec-
essary, trying harder than necessary. It is essential for great
programming teams, too. Hustle provides the cushion, the reserve
capacity, that enables a team to cope with routine mishaps, to
156 Hatching a Catastrophe
anticipate and forfend minor calamities. The calculated response,
the measured effort, are the wet blankets that dampen hustle. As
we have seen, one must get excited about a one-day slip. Such are
the elements of catastrophe.
But not all one-day slips are equally disastrous. So some calcu-
lation of response is necessary, though hustle be dampened. How
does one tell which slips matter? There is no substitute for a PERT
chart or a critical-path schedule. Such a network shows who waits
for what. It shows who is on the critical path, where any slip
moves the end date. It also shows how much an activity can slip
before it moves into the critical path.
The PERT technique, strictly speaking, is an elaboration of
critical-path scheduling in which one estimates three times for
every event, times corresponding to different probabilities of
meeting the estimated dates. I do not find this refinement to be
worth the extra effort, but for brevity I will call any critical path
network a PERT chart.
The preparation of a PERT chart is the most valuable part of
its use. Laying out the network, identifying the dependencies, and
estimating the legs all force a great deal of very specific planning
very early in a project. The first chart is always terrible, and one
invents and invents in making the second one.
As the project proceeds, the PERT chart provides the answer
to the demoralizing excuse, 'The other piece is late anyhow." It
shows how hustle is needed to keep one's own part off the critical
path, and it suggests ways to make up the lost time in the other
Under the Rug
When a first-line manager sees his small team slipping behind, he
is rarely inclined to run to the boss with this woe. The team might
be able to make it up, or he should be able to invent or reorganize
to solve the problem. Then why worry the boss with it? So far, so
Under the Rug 157
good. Solving such problems is exactly what the first-line manager
is there for. And the boss does have enough real worries demand-
ing his action that he doesn't seek others. So all the dirt gets swept
under the rug.
But every boss needs two kinds of information, exceptions to
plan that require action and a status picture for education.^ For
that purpose he needs to know the status of all his teams. Getting
a true picture of that status is hard.
The first-line manager's interests and those of the boss have
an inherent conflict here. The first-line manager fears that if he
reports his problem, the boss will act on it. Then his action will
preempt the manager's function, diminish his authority, foul up
his other plans. So as long as the manager thinks he can solve it
alone, he doesn't tell the boss.
Two rug-lifting techniques are open to the boss. Both must be
used. The first is to reduce the role conflict and inspire sharing of
status. The other is to yank the rug back.
Reducing the role conflict. The boss must first distinguish be-
tween action information and status information. He must disci-
pline himself not to act on problems his managers can solve, and
never to act on problems when he is explicitly reviewing status. I
once knew a boss who invariably picked up the phone to give
orders before the end of the first paragraph in a status report. That
response is guaranteed to squelch full disclosure.
Conversely, when the manager knows his boss will accept
status reports without panic or preemption, he comes to give hon-
This whole process is helped if the boss labels meetings, re-
views, conferences, as status-review meetings versus problem-action
meetings, and controls himself accordingly. Obviously one may
call a problem-action meeting as a consequence of a status meet-
ing, if he believes a problem is out of hand. But at least everybody
knows what the score is, and the boss thinks twice before grabbing
158 Hatching a Catastrophe
Yanking the rug off. Nevertheless, it is necessary to have review
techniques by which the true status is made known, whether
cooperatively or not. The PERT chart with its frequent sharp
milestones is the basis for such review. On a large project one may
want to review some part of it each week, making the rounds once
a month or so.
A report showing milestones and actual completions is the key
document. Figure 14.1 shows an excerpt from such a report. This
report shows some troubles. Specifications approval is overdue on
several components. Manual (SLR) approval is overdue on an-
other, and one is late getting out of the first state (Alpha) of the
independently conducted product test. So such a report serves as
an agenda for the meeting of 1 February. Everyone knows the
questions, and the component manager should be prepared to
explain why it's late, when it will be finished, what steps he's
taking, and what help, if any, he needs from the boss or collateral
V. Vyssotsky of Bell Telephone Laboratories adds the follow-
/ have found it handy to carry both "scheduled" and "estimated"
dates in the milestone report. The scheduled dates are the property of
the project manager and represent a consistent work plan for the
project as a whole, and one which is a priori a reasonable plan. The
estimated dates are the property of the lowest level manager who has
cognizance over the piece of work in question, and represents his best
judgment as to when it will actually happen, given the resources he
has available and when he received (or has commitments for delivery
of) his prerequisite inputs. The project manager has to keep his fingers
off the estimated dates, and put the emphasis on getting accurate,
unbiased estimates rather than palatable optimistic estimates or self-
protective conservative ones. Once this is clearly established in every-
one's mind, the project manager can see quite a ways into the future
where he is going to be in trouble if he doesn 't do something. *
Under the Rug 159
ifiit) mm intn mm mm m
\S SN SN NS SS N
mcM mtM mN m<M mtvi m
WSNNN SN VSV
— t\j — (\j M(M _(v — n —
oo oo oo oo oo oo oo oo oo oo
>« UJ u< o< u< u< u< u< u
3(u— cDUi ♦« ** *« «m *« *♦ •«* ♦♦ ** torn
Hm • <> SV SN NS SS SS SS SN ss ss ss
< m _IO nOJ N* N«0 (MB oo MO (MO n (D SCO CM*
►•♦o — K — — —— —o o- wrt — n -n
« _i<a ss ss ss ss ss ss ss
m> a>a -- mm -<m <m- -- -« -- -- -n -- -n
vaa «<< — "" —- —— — o -— —— -— ->- — — oo — —
zma UJ o u u< u< u u u u< o u
3UUJ muj ♦m ♦m <fm ♦m «m «m *m *m ♦m «io
mou. <> ss ss ss ss ss ss ss ss ss ss
or u)_io n« — M mo om o>— o- n- mo oo« « —
oau. u — a — — MM -<M no -"— -— —— — <m oo — —
lO o uj<a ss ss ss ss ss ss ss ss ss ss
row a>a o- o— o-< a-- o— o— o- o- n— o —
som (/)<< -o —o — o oo -o -<o — o — o —o —o
KU UJIU uuo uuuu uuuuu uu
WZ > JO
>•« — (DUJ ♦«•* <»«** ««'««« «<f
W_l HO SSS SSSS SSSSS SN
u_io coca ao«aoa> eoeoeomm <ooo
O UJuQC (MN(\i (M(VI(M(M (VJNNOO (\J(\J
« -)<a SSS SSSS SSSSS ss
D - -
160 Hatching a Catastrophe
The preparation of the PERT chart is a function of the boss
and the managers reporting to him. Its updating, revision, and
reporting requires the attention of a small (one to three man) staff
group which serves as an extension of the boss. Such a Plans and
Controls team is invaluable for a large project. It has no authority
except to ask all the line managers when they will have set or
changed milestones, and whether milestones have been met. Since
the Plans and Controls group handles all the paperwork, the bur-
den on the line managers is reduced to the essentials — making the
We had a skilled, enthusiastic, and diplomatic Plans and Con-
trols group, run by A. M. Pietrasanta, who devoted considerable
inventive talent to devising effective but unobtrusive control
methods. As a result, I found his group to be widely respected and
more than tolerated. For a group whose role is inherently that of
an irritant, this is quite an accomplishment.
The investment of a modest amount of skilled effort in a Plans
and Controls function is very rewarding. It makes far more differ-
ence in project accomplishment than if these people worked di-
rectly on building the product programs. For the Plans and
Controls group is the watchdog who renders the imperceptible
delays visible and who points up the critical elements. It is the
early warning system against losing a year, one day at a time.
The Other Face
The Other Face
What we do not understand we do not possess.
O give me commentators plain,
Who with no deep researches vex the brain.
A reconstruction of Stonehenge, the world's largest undocumented
The Bettman Archive
164 The Other Face
A computer program is a message from a man to a machine. The
rigidly marshaled syntax and the scrupulous definitions all exist
to make intention clear to the dumb engine.
But a written program has another face, that which tells its
story to the human user. For even the most private of programs,
some such communication is necessary; memory will fail the au-
thor-user, and he will require refreshing on the details of his
How much more vital is the documentation for a public pro-
gram, whose user is remote from the author in both time and
space! For the program product, the other face to the user is fully
as important as the face to the machine.
Most of us have quietly excoriated the remote and anonymous
author of some skimpily documented program. And many of us
have therefore tried to instill in new programmers an attitude
about documentation that would inspire for a lifetime, overcom-
ing sloth and schedule pressure. By and large we have failed. I
think we have used wrong methods.
Thomas J. Watson, Sr. told the story of his first experience as
a cash register salesman in upstate New York. Charged with en-
thusiasm, he sallied out with his wagon loaded with cash registers.
He worked his territory diligently but without selling a one.
Downcast, he reported to his boss. The sales manager listened a
while, then said, ''Help me load some registers into the wagon,
harness the horse, and let's go again." They did, and the two called
on customer after customer, with the older man showing how to sell
cash registers. All evidence indicates that the lesson took.
For several years 1 diligently lectured my software engineering
class on the necessity and propriety of good documentation, ex-
horting them ever more fervently and eloquently. It didn't work.
1 assumed they had learned how to document properly and were
failing from lack of zeal. Then I tried loading some cash registers
into the wagon; i.e., showing them how the job is done. This has
been much more successful. So the remainder of this essay will
downplay exhortation and concentrate on the "how" of good
What Documentation Is Required? 165
What Documentation Is Required?
Different levels of documentation are required for the casual user
of a program, for the user who must depend upon a program, and
for the user who must adapt a program for changes in circum-
stance or purpose.
To use a program. Every user needs a prose description of the
program. Most documentation fails in giving too little overview.
The trees are described, the bark and leaves are commented, but
there is no map of the forest. To write a useful prose description,
stand way back and come in slowly:
1. Purpose. What is the main function, the reason for the pro-
2. Environment. On what machines, hardware configurations,
and operating system configurations will it run?
3. Domain and range. What domain of input is valid? What range
of output can legitimately appear?
4. Functions realized and algorithms used. Precisely what does it do?
5. Input-output formats, precise and complete.
6. Operating instructions, including normal and abnormal ending
behavior, as seen at the console and on the outputs.
7. Options. What choices does the user have about functions?
Exactly how are those choices specified?
8. Running time. How long does it take to do a problem of speci-
fied size on a specified configuration?
9. Accuracy and checking. How precise are the answers expected
to be? What means of checking accuracy are incorporated?
Often all this information can be set forth in three or four
pages. That requires close attention to conciseness and precision.
Most of this document needs to be drafted before the program is
written, for it embodies basic planning decisions.
To believe a program. The description of how it is used must be
supplemented with some description of how one knows it is work-
ing. This means test cases.
166 The Other Face
Every copy of a program shipped should include some small
test cases that can be routinely used to reassure the user that he
has a faithful copy, accurately loaded into the machine.
Then one needs more thorough test cases, which are normally
run only after a program is modified. These fall into three parts of
the input data domain:
1. Mainline cases that test the program's chief functions for com-
monly encountered data.
2. Barely legitimate cases that probe the edge of the input data
domain, ensuring that largest possible values, smallest possi-
ble values, and all kinds of valid exceptions work.
3. Barely illegitimate cases that probe the domain boundary from
the other side, ensuring that invalid inputs raise proper diag-
To modify a program. Adapting a program or fixing it requires
considerably more information. Of course the full detail is re-
quired, and that is contained in a well-commented listing. For the
modifier, as well as the more casual user, the crying need is for a
clear, sharp overview, this time of the internal structure. What are
the components of such an overview?
1. A flow chart or subprogram structure graph. More on this
2. Complete descriptions of the algorithms used, or else refer-
ences to such descriptions in the literature.
3. An explanation of the layout of all files used.
4. An overview of the pass structure — the sequence in which
data or programs are brought from tape or disk — and what is
accomplished on each pass.
5. A discussion of modifications contemplated in the original
design, the nature and location of hooks and exits, and discur-
sive discussion of the ideas of the original author about what
modifications might be desirable and how one might proceed.
His observations on hidden pitfalls are also useful.
The Flow-Chart Curse 167
The Flow-Chart Curse
The flow chart is a most thoroughly oversold piece of program
documentation. Many programs don't need flow charts at all; few
programs need more than a one-page flow chart.
Flow charts show the decision structure of a program, which
is only one aspect of its structure. They show decision structure
rather elegantly when the flow chart is on one page, but the over-
Fig. 15.1 A program structure graph. (Courtesy of W. V. Wright)
168 The Other Face
view breaks down badly when one has multiple pages, sewed
together with numbered exits and connectors.
The one-page flow chart for a substantial program becomes
essentially a diagram of program structure, and of phases or steps.
As such it is very handy. Figure 15.1 shows such a subprogram
Of course such a structure graph neither follows nor needs the
painfully wrought ANSI flow-charting standards. All the rules on
box shapes, connectors, numbering, etc. are needed only to give
intelligibility to detailed flow charts.
The detailed blow-by-blow flow chart, however, is an obso-
lete nuisance, suitable only for initiating beginners into algorith-
mic thinking. When introduced by Goldstine and von Neumann,^
the little boxes and their contents served as a high-level language,
grouping the inscrutable machine-language statements into clus-
ters of significance. As Iverson early recognized,^ in a systematic
high-level language the clustering is already done, and each box
contains a statement (Fig. 15.2). Then the boxes themselves
become no more than a tedious and space-hogging exercise in
drafting; they might as well be eliminated. Then nothing is left but
the arrows. The arrows joining a statement to its successor are
redundant; erase them. That leaves only GO TO's. And if one
follows good practice and uses block structure to minimize GO
TO's, there aren't many arrows, but they aid comprehension im-
mensely. One might as well draw them on the listing and eliminate
the flow chart altogether.
In fact, flow charting is more preached than practiced. I have
never seen an experienced programmer who routinely made de-
tailed flow charts before beginning to write programs. Where or-
ganization standards require flow charts, these are almost
invariably done after the fact. Many shops proudly use machine
programs to generate this ''indispensable design tool" from the
completed code. I think this universal experience is not an embar-
rassing and deplorable departure from good practice, to be ac-
knowledged only with a nervous laugh. Instead it is the
The Flow-Chart Curse 169
application of good judgment, and it teaches us something about
the utiHty of flow charts.
The Apostle Peter said of new Gentile converts and the Jewish
law, ''Why lay a load on [their] backs which neither our ancestors
nor we ourselves were able to carry?" (Acts 15:10, TEV). I would
say the same about new programmers and the obsolete practice of
A basic principle of data processing teaches the folly of trying to
maintain independent files in synchronism. It is far better to com-
bine them into one file with each record containing all the infor-
mation both files held concerning a given key.
Yet our practice in programming documentation violates
our own teaching. We typically attempt to maintain a machine-
readable form of a program and an independent set of human-
readable documentation, consisting of prose and flow charts.
The results in fact confirm our teachings about the folly of
separate files. Program documentation is notoriously poor, and its
maintenance is worse. Changes made in the program do not
promptly, accurately, and invariably appear in the paper.
The solution, I think, is to merge the files, to incorporate the
documentation in the source program. This is at once a powerful
incentive toward proper maintenance, and an insurance that the
documentation will always be handy to the program user. Such
programs are called self- documenting.
Now clearly this is awkward (but not impossible) if flow
charts are to be included. But grant the obsolescence of flow charts
and the dominant use of high-level language, and it becomes
reasonable to combine the program and the documentation.
The use of a source program as a documentation medium
imposes some constraints. On the other hand, the intimate avail-
ability of the source program, line by line, to the reader of the
documentation makes possible new techniques. The time has
170 The Other Face
» — — «^— — — war
9- Z T X > T ZZ> —
,» 9... ».«.~>.. _»>
ff- C ^ l«J •J Kl
ec ' ec tc ' at vL'tttctfeC^tCtC'tC
UJ \Xi ^1 UJ Ui UJUJUJ UJ UJUU Hi
I — XI — I I — III — I — IX — X
Self-Documenting Programs 171
172 The Other Face
come to devise radically new approaches and methods for program
As a principal objective, we must attempt to minimize the
burden of documentation, the burden neither we nor our prede-
cessors have been able to bear successfully.
An approach. The first notion is to use the parts of the program
that have to be there anyway, for programming language reasons,
to carry as much of the documentation as possible. So labels,
declaration statements, and symbolic names are all harnessed to
the task of conveying as much meaning as possible to the reader.
A second notion is to use space and format as much as possible
to improve readability and show subordination and nesting.
The third notion is to insert the necessary prose documenta-
tion into the program as paragraphs of comment. Most programs
tend to have enough line-by-line comments; those programs pro-
duced to meet stiff organizational standards for ''good documenta-
tion'' often have too many. Even these programs, however, are
usually deficient in the paragraph comments that really give intel-
ligibility and overview to the whole thing.
Since the documentation is built into the structure, naming,
and formats of the program, much of it must be done when the
program is first written. But that is when it should be written. Since
the self-documentation approach minimizes extra work, there are
fewer obstacles to doing it then.
Some techniques. Figure 15.3 shows a self-documenting PL/I
program.^ The numbers in the circles are not part of it; they are
meta-documentation keyed to the discussion.
1. Use a separate job name for each run, and maintain a run log
showing what was tried, when, and the results. If the name is
composed of a mnemonic part (here QLT) and a numerical
suffix (here 4), the suffix can be used as a run number, tying
listings and log together. This technique requires a new job
card for each run, but they can be made up in batches, dupli-
cating the common information.
Fig. 15.3 A self-documenting program. ►
(T)//QLTU JOB ...
(2)qLTSRT7: procedure (V) :
/*A SORT SUBROUTINE FOR 2500 6
/♦SEPARATELY COMPILED, NOT-MAI
/♦THE SORT AL30BITHfl FOLLOHS B
/♦PROGRAM 7.23, P. 350. THAT
/♦ STEPS 2-12 ARE SIMPLIFIED
/♦ STEP 18 IS EXPANDED TO HAN
THE WHOLE FIELD IS USED AS
MINUS INFINirif IS REPrfESEN
PLUS INFINITY IS REPRESENT
THF STATEMENT NUMBERS IN P
LABELS OF THIS PfiOr.RAM.
AN IF-THEN-ELSE CONSTRUCTI
♦ ♦♦♦♦♦♦♦♦♦♦♦ ♦♦♦♦♦♦♦♦♦♦«^**«*^**4>*** «•«**••*««
-BYTE FIELDS, PASSED AS THE VECTOR V. A
N PROCEDURE, WHICH MUST USE AUTOnATIC CORE
/♦TO CHANGE THE DIMENSION OF T
/♦INITIALIZATION OF T. IF THE
/♦A MORE GENERAL VERSION MOULD
/♦THE PASSED INPUT VECTOR IS R
ROOKS AND IVERSON, AUTOMATIC DATA PROCESSING,
ALGORITHM IS REVISED A3 FOLLOWS:
DLE EXPLICIT INDEXING OF THE OUTPUT VECTOR.
THE SORT KEY.
TED BY ZEROS.
ED BY ONES.
ROG. 7.23 ARE REFLECTED IN THE STATEMENT
ON REQUIRES REPETITION OF A FEW LINES.
HE VECTOR TO BE SORTED, ALWAYS CHANGE THE
SIZE EXCEEDS 4096, CHANGE THE SIZE OF r,TOO.
PARAMETERIZE THE DIMENSION OF V.
EPLACED BY THE REORDERED OUTPUT VECTOR.
(?)/♦ LEGEND (ZERO-ORIGIN INDEXING)
K) BINARY FIXED,
PINF) BIT (U8),
/♦I»<DEX FOR INITIALIZING T
/♦INDEX OF ITEM TO BE REPLACED
/♦INITIAL INDEX OF BRANCHES FROM NODE I
/♦INDEX IN OUTPUT VECTOR
/♦PASSED VECTOR TO BE SORTED AMD RETURNED
T (0:8190) BIT (48); /♦WORKSPACE CONSISTING OF VECTOR TO BE SORTED, FILLED^/
/♦OUT WITH INFINITIES, PRECEDED BY LOWER LEVELS ♦/
/♦FILLED UP WITH MINUS INFINITIES ♦/
/* NOH INITIALIZATION TO FILL DUMMY LEVELS,
/* LEVEL AS REQUIRED.
TOP LEVEL, AND UNUSED PART OF TOP*/
DO L= TO 4094
DO L= TO 2499
DO L=6595 TO 3190
T(L) = MINF: END;
T(L+4095) = V(L) ; END;
T(L) = PINF; END;
(J) <= T(Jf1)
T(I) = T(J)
/♦SET J TO SCAN BRANCHES PROM NODE I.
/♦PICK SMALLER BRANCH
= PINF THEN 30 TO K16: /♦IF INFINITY, REPLACEMENT
/♦ IS FINISHED
/♦SET INDEX FOR HIGHER LEVEL
IF T(I) = PINF THEN GO TO K16: /*
I = J+1: /♦
2^1 < 8191 THEM GO TO K3; /♦GO BACK IF NOT ON TOP LEVEL
) = PINF; /♦IF TOP LEVEL, FILL WITH INFINITY
T(0) = PINF THEN RETURN;
T(0) = MINF THEN GO TO Kl;
= T(0) : GO TO Kl; (g)
/♦TEST END OF SORT
/♦FLUSH our INITIAL DUMMIES
/♦STEP STORAGE INDEX
/♦STORE OUTPUT ITEM
* °° H
< f" 1
+ oo J 1
< ' '
174 The Other Face
2. Use a program name that is mnemonic but also contains a
version identifier. That is, assume there will be several ver-
sions. Here the index is the low order digit of the year 1967.
3. Incorporate the prose description as comments to PROCE-
4. Refer to standard literature to document basic algorithms
wherever possible. This saves space, usually points to a much
fuller treatment than one would provide, and allows the
knowledgeable reader to skip it with confidence that he un-
5. Show the relationship to the book algorithm:
a) changes b) specialization c) representation
6. Declare all variables. Use mnemonic names. Use comments to
convert DECLARE into a complete legend. Note that it already
contains names and structural descriptions, it needs only to be
augmented with descriptions of purpose. By doing so here, one
can avoid repeating the names and structural descriptions in
a separate treatment.
7. Mark the initialization by a label.
8. Label statements in groups to show correspondences to the
statements in the algorithm description in the literature.
9. Use indenting to show structure and grouping.
10. Add logical flow arrows to the listing by hand. They are very
helpful in debugging and changing. They may be incorporated
in the right margin of the comments space, and made part of
the machine-readable text.
11. Use line comments or remark anything that is not obvious. If
the techniques above have been used, these will be short and
fewer in number than is customary.
12. Put multiple statements on one line, or one statement on sev-
eral lines to match thought-grouping and to show correspon-
dence to other algorithm description.
Why not? What are the drawbacks of such an approach to docu-
mentation? There are several, which have been real but are becom-
ing imaginary with changing times.
Self-Documenting Programs 175
The most serious objection is the increase in the size of the
source code that must be stored. As the discipline moves more and
more toward on-line storage of source code, this has become a
growing consideration. I find myself being briefer in comments to
an APL program, which will live on a disk, than on a PL/I one that
I will store as cards.
Yet simultaneously we are moving also toward on-line storage
of prose documents for access and for updating via computerized
text-editing. As shown above, amalgamating prose and program
reduces the total number of characters to be stored.
A similar answer applies to the argument that self-document-
ing programs require more keystrokes. A typed document requires
at least one keystroke per character per draft. A self-documenting
program has fewer total characters and also fewer strokes per
character, since drafts aren't retyped.
How about flow charts and structure graphs? If one uses only
a highest-level structure graph, it might safely be kept as a sepa-
rate document, for it is not subject to frequent change. But it can
certainly be incorporated into the source program as a comment,
and that seems wise.
To what extent are the techniques used above applicable to
assembly language programs? I think the basic approach of self-
documentation is thoroughly applicable. Space and formats are
less free, and thus cannot be so flexibly used. Names and structural
declarations can surely be exploited. Macros can help a great deal.
The extensive use of paragraph comments is good practice in any
But the self-documentation approach is stimulated by the use
of high-level languages and finds its greatest power and its great-
est justification in high-level languages used with on-line systems,
whether batch or interactive. As I have argued, such languages and
systems help programmers in very powerful ways. Since machines
are made for people, not people for machines, their use makes
every form of sense, economic and human.
The tar pit of software engineering will continue to be sticky for
a long time to come. One can expect the human race to continue
attempting systems just within or just beyond our reach; and
software systems are perhaps the most intricate and complex of
man's handiworks. The management of this complex craft will
demand our best use of new languages and systems, our best
adaptation of proven engineering management methods, liberal
doses of common sense, and a God-given humility to recognize
our fallibility and limitations.
Notes and References
1. Ershov considers this not only a woe, but also a part of the joy.
A. P. Ershov, ''Aesthetics and the human factor in program-
ming," CACM, 15, 7 (July, 1972), pp. 501-505.
1. V.A. Vyssotsky of Bell Telephone Laboratories estimates that
a large project can sustain a manpower buildup of 30 percent
per year. More than that strains and even inhibits the evolu-
tion of the essential informal structure and its communication
pathways discussed in Chapter 7.
F. J. Corbato of MIT points out that a long project must antici-
pate a turnover of 20 percent per year, and these must be both
technically trained and integrated into the formal structure.
2. C. Portman of International Computers Limited says, "When
everything has been seen to work, all integrated, you have four more
months work to do. " Several other sets of schedule divisions are
given in Wolverton, R. W., 'The cost of developing large-
scale software," IEEE Trans, on Computers, C-23, 6 (June, 1974)
3. Figures 2.5 through 2.8 are due to Jerry Ogdin, who in quoting
my example from an earlier publication of this chapter much
improved its illustration. Ogdin, J. L., 'The Mongolian hordes
versus superprogrammer," Infosystems (December, 1972), pp.
180 Notes and References
1. Sackman, H., W. J. Erikson, and E. E. Grant, ''Exploratory
experimental studies comparing online and offline program-
ming performance/' CACM, 11, 1 (January, 1968), pp. 3-11.
2. Mills, H., "Chief programmer teams, principles, and proce-
dures," IBM Federal Systems Division Report FSC 71-5108,
Gaithersburg, Md., 1971.
3. Baker, F. T., "Chief programmer team management of produc-
tion programming," IBM Sys. /, 11, 1 (1972).
1. Eschapasse, M,, Reims Cathedral, Caisse Nationale des Monu-
ments Historiques, Paris, 1967.
2. Brooks, F. P., "Architectural philosophy," in W. Buchholz
(ed.), A Computer System. New York: McGraw-Hill, 1962.
3. Blaauw, G. A., "Hardware requirements for the fourth genera-
tion," in F. Gruenberger (ed.). Fourth Generation Computers.
Englewood Cliffs, N.J.: Prentice-Hall, 1970.
4. Brooks, F. P., and K. E. Iverson, Automatic Data Processing,
System/ 360 Edition. New York: Wiley, 1969, Chapter 5.
5. Glegg, G. L., The Design of Design. Cambridge: Cambridge
Univ. Press, 1969, says 'At first sight, the idea of any rules or
principles being superimposed on the creative mind seems more likely
to hinder than to help, hut this is quite untrue in practice. Disciplined
thinking focusses inspiration rather than blinkers it. "
6. Conway, R. W., "The PL/C Compiler," Proceedings of a Conf
on Definition and Implementation of Universal Programming Lan-
guages. Stuttgart, 1970.
7. For a good discussion of the necessity for programming tech-
nology, see C. H. Reynolds, "What's wrong with computer
Notes and References 181
programming management?" in G. F. Weinwurm (ed.). On the
Management of Computer Programming. Philadelphia: Auerbach,
1971, pp. 35-42.
1. Strachey, C, ''Review of Planning a Computer System," Comp.
/, 5, 2 (July, 1962), pp. 152-153.
2. This applies only to the control programs. Some of the com-
piler teams in the OS/360 effort were building their third or
fourth systems, and the excellence of their products shows it.
3. Shell, D. L., 'The Share 709 system: a cooperative effort";
Greenwald, E. D., and M. Kane, 'The Share 709 system: pro-
gramming and modification"; Boehm, E. M., and T. B. Steel,
Jr., "The Share 709 system: machine implementation of sym-
bohc programming"; all in J ACM, 6, 2 (April, 1959), pp. 123-
1. Neustadt, R. E., Presidential Power. New York: Wiley, 1960,
2. Backus, J. W., "The syntax and semantics of the proposed
international algebraic language." Proc. Intl. Conf. Inf. Proc.
UNESCO, Paris, 1959, published by R. Oldenbourg, Munich,
and Butterworth, London. Besides this, a whole collection of
papers on the subject is contained in T.B. Steel, Jr. (ed.). Formal
Language Description Languages for Computer Programming. Am-
sterdam: North Holland, (1966).
3. Lucas, P., and K. Walk, "On the formal description of PL/I,"
Annual Review in Automatic Programming Language. New York:
Wiley, 1962, Chapter 2, p. 2.
4. Iverson, K. E., A Programming Language. New York: Wiley,
1962, Chapter 2.
182 Notes and References
5. Falkoff, A. D., K. E. Iverson, E. H. Sussenguth, "A formal
description of System/360/' IBM Systems Journal 3, 3 (1964),
6. Bell, C. G., and A. Newell, Computer Structures. New York.
McGraw-Hill, 1970, pp. 120-136, 517-541.
7. Bell, C. G., private communication.
1. Parnas, E. L., 'Information distribution aspects of design
methodology,'' Carnegie- Mellon Univ., Dept. of Computer
Science Technical Report, February, 1971.
2. Heinlein, R. A., The Man Who Sold the Moon. New York:
Signet, 1951, pp. 103-104. Reprinted by permission of the
author, copyright 1950 by Robert A. Heinlein.
1. Sackman, H., W. J. Erikson, and E. E. Grant, "Exploratory
experimentation studies comparing online and offline pro-
gramming performance," CACM, 11, 1 (January, 1968), pp.
2. Nanus, B., and L. Parr, "Some cost contributors to large-scale
programs," AFIPS Proc. SJCQ 25 (Spring, 1964), pp. 239-248.
3. Weinwurm, G. P., "Research in the management of computer
programming," Report SP-2059, System Development Corp.,
Santa Monica, 1965.
4. Morin, L. H., "Estimation of resources for computer program-
ming projects," M. S. thesis, Univ. of North Carolina, Chapel
5. Portman, C, private communication.
6. An unpublished 1964 study by E. P. Bardain shows program-
mers realizing 27 percent productive time. (Quoted by D.B.
Notes and References 183
Mayer and A. W. Stalnaker, ''Selection and evaluation of com-
puter personnel/' Proc. 23rd ACM Conf., 1968, p. 661.)
7. Aron, J., private communication.
8. Paper given at a panel session and not included in the AFIPS
9. Wolverton, R. W., 'The cost of developing large-scale soft-
ware," IEEE Trans, on Computers, C-23, 6 (June, 1974) pp.
615-636. This important recent paper contains data on many
of the issues of this chapter, as well as confirming the produc-
10. Corbato, F. J., "Sensitive issues in the design of multi-use
systems," lecture at the opening of the Honeywell EDP Tech-
nology Center, 1968.
11. W. M. Taliaffero also reports a constant productivity of 2400
statements/year in assembler, Fortran, and Cobol. See
"Modularity. The key to system growth potential," Software,
1, 3 (July 1971) pp. 245-257.
12. E. A. Nelson's System Development Corp. Report TM-3225,
Management Handbook for the Estimation of Computer Program-
ming Costs, shows a 3-to-l productivity improvement for
high-level language (pp. 66-67), although his standard devia-
tions are wide.
1. Brooks, F. P. and K. E. Iverson, Automatic Data Processing, Sys-
tem/360 Edition. New York: Wiley, 1969, Chapter 6.
2. Knuth, D. E., The Art of Computer Programming, Vols. 1-3.
Reading, Mass.: Addison-Wesley, 1968, ff.
1. Conway, M. E., "How do committees invent?" Dfl/flmfl//^^7, 14,
4 (April, 1968), pp. 28-31.
184 Notes and References
1. Speech at Oglethorpe University, May 22, 1932.
2. An illuminating account of Multics experience on two succes-
sive systems is in F. J. Corbato, J. H. Saltzer, and C. T. Clingen,
''Multics— the first seven years," AFIPS Proc SJCC. 40 (1972),
3. Cosgrove, J., ''Needed: a new planning framework," Datama-
Hon, 17, 23 (Dec, 1971), pp. 37-39.
4. The matter of design change is complex, and I oversimplify
here. See J. H. Saltzer, "Evolutionary design of complex sys-
tems," in D. Eckman (ed.). Systems: Research and Design. New
York: Wiley, 1961. When all is said and done, however, I still
advocate building a pilot system whose discarding is planned.
5. Campbell, E., "Report to the AEC Computer Information
Meeting," December, 1970. The phenomenon is also discussed
by J. L. Ogdin in "Designing reliable software," Datamation,
18, 7 (July, 1972), pp. 71-78. My experienced friends seem
divided rather evenly as to whether the curve finally goes
6. Lehman, M., and L. Belady, "Programming system dynamics,"
given at the ACM SIGOPS Third Symposium on Operating
System Principles, October, 1971.
7. Lewis, C. S., Mere Christianity. New York: Macmillan, 1960,
1. See also J. W. Pomeroy, "A guide to programming tools and
techniques," IBM Sys. /, 11, 3 (1972), pp. 234-254.
2. Landy, B., and R. M. Needham, "Software engineering tech-
niques used in the development of the Cambridge Multiple-
Access System," Software, 1, 2 (April, 1971), pp. 167-173.
Notes and References 185
Corbato, F. J., 'TL/I as a tool for system programming/' Data-
mation, 15, 5 (May, 1969), pp. 68-76.
Hopkins, M., ''Problems of PL/I for system programming,"
IBM Research Report RC 3489, Yorktown Heights, N.Y., Au-
gust 5, 1971.
Corbato, F. J., J. H. Saltzer, and C. T. Clingen, "Multics— the
first seven years," AFIPS Proc S]CC, 40 (1972), pp. 571-582.
"Only a half-dozen areas which were written in PL /I have been
recoded in machine language for reasons of squeezing out the utmost
in performance. Several programs, originally in machine language,
have been recoded in PL /I to increase their maintainability. "
To quote Corbato's paper cited in reference 3: 'PL/ I is here now
and the alternatives are still untested. " But see a quite contrary
view, well-documented, in Henricksen, J. O. and R. E. Mer-
win, "Programming language efficiency in real-time software
systems," AFIPS Proc SJCC, 40 (1972) pp. 155-161.
Not all agree. Harlan Mills says, in a private communication,
"h4y experience begins to tell me that in production programming the
person to put at the terminal is the secretary. The idea is to make
programming a more public practice, under common scrutiny of many
team members, rather than a private art. "
Harr, J., "Programming Experience for the Number 1 Elec-
tronic Switching System," paper given at the 1969 SJCC.
1. Vyssotsky, V. A., "Common sense in designing testable soft-
ware," lecture at The Computer Program Test Methods Sym-
posium, Chapel Hill, N.C., 1972. Most of Vyssotsky's lecture
is contained in Hetzel, W. C. (ed.). Program Test Methods.
Englewood Cliffs, N.J.: Prentice-Hall, 1972, pp. 41-47.
186 Notes and References
2. Wirth, N., 'Trogram development by stepwise refinement/'
CACM 14, 4 (April, 1971), pp. 221-227. See also Mills, H.
'Top-down programming in large systems,'' in R. Rustin (ed.).
Debugging Techniques in Large Systems. Englewood Cliffs, N.J.:
Prentice-Hall, 1971, pp. 41-55 and Baker, F. T., "System qual-
ity through structured programming," AFIPS Proc FJCC, 41-1
(1972), pp. 339-343.
3. Dahl, O. J., E. W. Dijkstra, and C. A. R. Hoare, Structured
Programming. London and New York: Academic Press, 1972.
This volume contains the fullest treatment. See also Dijkstra's
germinal letter, "GOTO statement considered harmful,"
CACM, 11, 3 (March, 1968), pp. 147-148.
4. Bohm, C, and A. Jacopini, "Flow diagrams, Turing machines,
and languages with only two formation rules," CACM, 9, 5
(May, 1966), pp. 366-371.
5. Codd, E. F., E. S. Lowry, E. McDonough, and C. A. Scalzi,
"Multiprogramming STRETCH: Feasibility considerations,"
CACM, 2, 11 (Nov., 1959), pp. 13-17.
6. Strachey, C, "Time sharing in large fast computers," Proc. Int.
Conf. on Info. Processing, UNESCO (June, 1959), pp. 336-341.
See also Codd's remarks on p. 341, where he reported progress
on work like that proposed in Strachey's paper.
7. Corbato, F. J., M. Merwin-Daggett, R. C. Daley, "An experi-
mental time-sharing system," AFIPS Proc. SJCC, 2, (1962), pp.
335-344. Reprinted in S. Rosen, Programming Systems and Lan-
guages. New York: McGraw-Hill, 1967, pp. 683-698.
8. Gold, M. M., "A methodology for evaluating time-shared
computer system usage," Ph.D. dissertation, Carnegie- Mellon
University, 1967, p. 100.
9. Gruenberger, F., "Program testing and validating," Datama-
tion, 14, 7, (July, 1968), pp. 39-47.
Notes and References 187
10. Ralston, A., Introduction to Programming and Computer Science.
New York: McGraw-Hill, 1971, pp. 237-244.
11. Brooks, F. P., and K. E. Iverson, Automatic Data Processing,
System/360 Edition. New York: Wiley, 1969, pp. 296-299.
12. A good treatment of development of specifications and of
system build and test is given by F. M. Trapnell, "A systematic
approach to the development of system programs,'' AFIPS Proc
SJCC 34 (1969) pp. 411-418.
13. A real-time system will require an environment simulator.
See, for example, M. G. Ginzberg, ''Notes on testing real-time
system programs," IBM Sys. J., 4, 1 (1965), pp. 58-72.
14. Lehman, M., and L. Belady, "Programming system dynamics,"
given at the ACM SIGOPS Third Symposium on Operating
System Principles, October, 1971.
1. See C. H. Reynolds, "What's wrong with computer program-
ming management?" in G. F. Weinwurm (ed.). On the Manage-
ment of Computer Programming. Philadelphia: Auerbach, 1971,
2. King, W. R., and T. A. Wilson, "Subjective time estimates in
critical path planning — a preliminary analysis," Mgt. Sci., 13,
5 (Jan., 1967), pp. 307-320, and sequel, W. R. King, D. M.
Witterrongel, K. D. Hezel, "On the analysis of critical path
time estimating behavior," Mgt. Sci., 14, 1 (Sept., 1967), pp.
3. For a fuller discussion, see Brooks, F. P., and K. E. Iverson,
Automatic Data Processing, System/ 3 60 Edition, New York:
Wiley, 1969, pp. 428-430.
4. Private communication.
188 Notes and References
1. Goldstine, H. H., and J. von Neumann, ''Planning and coding
problems for an electronic computing instrument/' Part II,
Vol. 1, report prepared for the U.S. Army Ordinance Depart-
ment, 1947; reprinted in J. von Neumann, Collected Works, A.
H. Taub (ed.). Vol. v.. New York: McMillan, pp. 80-151.
2. Private communication, 1957. The argument is pubhshed in
Iverson, K. E., 'The Use of APL in Teaching," Yorktown,
N.Y.: IBM Corp., 1969.
3. Another list of techniques for PL/I is given by A. B. Walter
and M. Bohl in "From better to best — tips for good program-
ming," Software Age, 3, 11 (Nov., 1969), pp. 46-50.
The same techniques can be used in Algol and even Fortran.
D. E. Lang of the University of Colorado has a Fortran format-
ting program called STYLE that accomplishes such a result.
See also D. D. McCracken and G. M. Weinberg, "How to write
a readable FORTRAN program," Datamation, 18, 10 (Oct.,
1972), pp. 73-77.
advancement, dual ladder of, 119
Algol, 34, 181, 188
Algol, 68, 44, 64
allocation, dynamic core, 57
APL (A Programming Language),
64, 98, 136, 175
architect, 37, 54, 62, 66, 79
architecture, 44, 143
archive, chronological, 33
aristocracy, 39, 44, 46
Aron, J., 90, 93, 183
ARPA network, 78
authority, 8, 80
Bach, J. S., 47
Backus, J. W., 64, 181
Backus-Naur Form, 64
Baker, F. T., 36, 180, 186
Bardain, E. F., 182
barrier, sociological, 119
Belady, L., 122, 123, 150, 184,
Bell, C. G., 64, 182
Bell Telephone Laboratories, viii,
90, 119, 133, 137, 142, 158,
Bengough, W., 107
Blaauw, G. A., 45, 49, 62, 63, 180
Boehm, E. M., 181
Bohl, M., 188
Bohm, C., 144, 186
Boudot-Lamotte, E., 40
Breughel, P., the Elder, 73
Brooks, F. P., Jr., 102, 180, 183,
Brooks's Law, 25
Buchholz, W., 180
budget, 6, 108, 110
bug, 142, 143
build-up, manpower, 179
Cambridge University, 133
Campbell, E., 121, 184
Canova, A., 153
Capp, A., 80
Carnegie- Mellon University, 78
Case, R. P., viii
CASE statement, 144
Cashman, T. J., 169
control of, 149
design, 166, 183
change summary, 77, 78
Note: Bold numerals indicate relatively substantial discussions of a topic.
chief programmer, 32, 180
clerk, program, 33
Clingen, C. T., 184, 185
Codd, E. F., 146, 186
committee, 79, 183
communication, 16, 17, 35, 54,
74, 78, 79, 88, 100, 111
compatibility, 63, 64, 68
compile-time operation, 66
component debugging, 144
component, dummy, 148
computer facility, 128
conceptual integrity, 42
control program, 91, 93, 180
convergence of debugging, 9
Conway, M. E., Ill, 183
Conway, R. W., 47, 180
Cooley, J. W., 102
Corbato, F. J., viii, 93, 146, 179,
183, 184, 185, 186
Cornell University, 47
Cosgrove, J., 117, 118, 184
court, for design disputes, 66
Crabbe, C, 163
creation, component stages, 15,
creative joy, 120
creative style, 47
creative work, 46
critical-path schedule, 89, 156,
Crockwell, D., 87
Crowley, W. R., 132
d'Orbais, J., 41
Dahl, O. J., 186
Daley, R. C, 186
data base, 108
data service, 131
date, estimated, 158
debugging, component 144
high-level language, 135
interactive, 34, 146
sequential nature of, 17
debugging aid, 128
DEC PDP-8, 64
dependability of debugging
description; see specification
difference in judgement, 35
difference of interest, 35
Digitek Corporation, 102
Dijkstra, E. W., 144, 186
director, technical, role of, 79
discipline, 46, 54, bb
Disk Operating System,
1410-7010, 56, 51, 99
display terminal, 78, 129
division of labor, 79
DO . . . WHILE, 144
documentation, 6, 32, 33, 122,
documentation system, 134
dummy component, 148
dump, memory, 133, 145
ease of use, 43, 98
Eckman, D., 184
editor, job description for, 33
text, 32, 34, 68, 128, 133, 134,
Electronic Switching System, 90
Engelbart, D.C., 78
environment, 6, 165
Erikson, W. J., 29, 30, 88, 180,
Ershov, A. P., viii, 179
Eschapasse, M., 180
estimating, 14, 21, 88, 109, 155,
Evans, B. O., v
Fagg, P., 24
Falkoff, A. D., 182
Farr, L., 88, 182
Fast Fourier Transform, 102
file, dummy, 148
flow arrow, 174
flow chart, 167
formal definition, 63
formal document. 111
formal progression of release, 133
formality, of written proposals,
Fortran, 45, 102, 188
Fortran, H., 99
Franklin, B. (Poor Richard), 87
Franklin, J. W., 134
Ginzberg, M. G., 187
Glegg, G. L., 180
GO TO, 170
Goethe, J. W. von, 163
Gold, M. M., 146, 186
Goldstine, H. H., 170
Grant, E. E., 29, 30, 88, 180, 182
Greenwald, E. D., 181
Gruenberger, F., 147, 180, 186
Hardy, H., 97
HaiT, J., viii, 90, 93, 137, 185
Heinlein, R., 81, 182
Henricksen, J. O., 185
Hetzel, W. C., 185
Hezel, K. D., 187
Hoare, C. A. R., 186
Hopkins, M., 185
IBM Corporation, viii, 90, 119
IBM Stretch computer, 44, 47, 55,
IBM System/360, 44, 45, 62, 64
Model 30, 45, 47
Model 65, 99
Model 75, 47
Model 165, 98
IBM System/ 360 Principles of
IBM 650, 43, 102
IBM 701, 131
IBM 704, 5b
IBM 709, 55
IBM 1401, 45, 65, 130
IBM 7090, 55, 64
IBSYS for the 7090, 56
idea, as stage of creation, 15
IF . . . THEN . . . ELSE, 144
implementation, 15, 45, 64, 143
implementations, multiple, 68
implementer, 47, 54, 62, 66
incorporation, direct, 66
input range, 6
input-output format, 165
integrity, conceptual, 35, 36, 42,
43, 62, 80, 142
interaction, as part of creation, 15
interaction, first of session, 146
interactive debugging, 34, 146
interactive programming, 136
interface, 6, 32, 62, 66, 79, 118,
International Computers Limited,
viii, 89, 133, 179
interpreter, for space-saving, 102
Iverson, K. E., 64, 102, 170, 180,
182, 183, 187
Jacopini, A., 144, 186
job name, 172
joys of the craft, 7
Kane, M., 181
Keys, W. J., 169
King, W. R., 187
Knight, C. R., 3
Knuth, D. E., 102, 183
Landy, B., 184
Lang, D. E., 188
language description, formal, 181
language, high-level, 118, 135,
143, 146, 183
language translator, 93
lawyer, language, 34
Lehman, M., 122, 123, 150, 184,
Lewis, C. S., 123, 184
library, macro, 34
linkage editor, 56
Locken, O. S., 76
Lowry, E. S., 186
Lucas, P., 181
magic, 7, 142
management information system,
Massachusetts Institute of
Technology, viii, 93, 121,
matrix-type organization, 79
Mayer, D. B., 183
McCracken, D. D., 188
McDonough, E., 186
Mealy, G., viii
medium of creation, 15
tractable, 7, 15, 117
meeting, problem action, 157
status review, 75, 157
memory use pattern, 129
Merwin, R. E., 185
Merwin- Daggett, M., 186
milestone, 22, 25, 154, 158
Mills, H., 32, 33, 180, 185
mini-decision, 63, 111
mnemonic name, 174
module, 101, 122, 143
modules, number of, 122
Mooers, C. N., 44
Moore, S. E., ix
Morin, L. H., 88, 182
MULTICS, 93, 136, 146, 183, 185
multiple implementations, 68
Nanus, B., 8.8, 182
Naur, P., 64
Needham, R. M., 184
Nelson, E. A., 183
nesting, as documentation aid,
network nature of
Newell, A., 64, 182
notebook, status, 33
objective, 8, 75, 108, 110, 117
cost and performance, 49
space and time, 49
obsolescence, 9, 26, 123
Ogdin, J. L., 179, 184
Oldenbourg, R., 181
operating system, 128
Operating System/360, 43, 45,
47, 56, 76, 93, 129
option, 101, 165
Orbais, J. d', 41
organization, 74, 78
organization chart, 108, 111
OS/ 3 60 Concepts and Facilities,
overlay, 56, 99, 129
Padegs, A., 62
Parnas, D. L., 78, 182
Pascal, B., 123
pass structure, 166
perfection, requirement for, 8
performance simulator, 134
PERT chart, 89, 156, 158
Peter the Apostle, 171
Piestrasanta, A. M., viii, 160
pilot plant, 116
Pisano, A., 127
PL/C compiler, 47, 180
PL/I, 32, 47, 64, 66, 93, 135, 172,
181, 184, 185, 188
Plans and Controls team, 160
playpen, 133, 149
policed system, 65
Pomeroy, J. W., 184
Poor Richard (Benjamin
Portman, C, viii, 89, 179, 182
procedure, catalogued, 34
producer, role in organization, 79
product, programming, 5
programming system, 4
product test, 69
productivity, programmer, 21, 30,
88, 94, 135, 179, 182
program clerk, 33
program library, 132
program maintenance, 120
program name, 174
program structure graph, 170
programming product, 5
programming system, 6
programming systems product, 4
Project Mercury Real-Time
promotion, in rank, 120
purple-wire technique, 149
purpose, of a program, 165
of a variable, 174
quantization, of change, 62, 118,
of demand for change, 117
raise in salary, 120
Ralston, A., 187
realization, step in creation, 49,
refinement step, 143
regenerative schedule disaster, 21
Reims Cathedral, 41
remote job entry, 58
representation, of information,
responsibility, versus authority, 8
Restaurant Antoine, 13
Reynolds, C. H., 180, 187
role conflict, reducing, 157
Roosevelt, F. D., 115
Rosen, S., 186
Rustin, R., 185
Ruth, G. H. (Babe), 87
Sackman, H., 29, 30, 88, 180,
Saltzer, J. H., 184, 185
Sayers, D., 15
scaffolding, 34, 148
scaling up, 36, 116
Scalzi, C. A., 186
schedule, 79, 108, 111, 154
scheduling, 14, 129
second-system effect, 51
self- documenting program, 118,
semantics, 44, 64, 66
Shakespeare, W., 141
Share Operating System for 709,
Shell, D. L., 181
side-effect, 65, 122
simulator, environment, 187
logic, 65, 131
size, program, 30, 98, 135, 175
size trimming, 129
Sloane, J. C, ix
Smith, S., 97
sociological barrier, 119
Sophocles, 153, 155
space allocation, 108, 111
space, program; see size, program
specialization of function, 35, 79
specification, architectural, 43
functional, 32, 49, 62, 75, 108,
testing the, 142
speed, program, 30, 98, 135
spiral, pricing- forecasting, 109
staff group, 79
Stalnaker, A. W., 183
Stanford Research Institute, 78
status control, 108
status report, 157
status review meeting, 157
status symbol, 81
Steel, T. B., Jr. 181
Strachey, C, 56, 146, 181, 186
Stretch Operating System, 56, 99
structured programming, 32, 144
superior- subordinate relationship,
supervisory program, 129, 146
surgical team, 27, 120
Sussenguth, E. H., 182
Swift, J., 115
synchronism in file, 171
syntax, 44, 64, 66
system debugging, 132, 147
system integration sublibrary, 133
system, large, 31
system test, 19, 122, 133, 147
systems product, programming, 4
Tacoma Narrows Bridge, 115
Taliaffero, W. M., 183
target machine, 129
task force, 79
team, small, sharp, 30
technical director, 79
technology, programming, 49,
102, 128, 180
telephone log, 68
test case, 6, 34, 147, 166
test-case generator, 128
test, component, 20
system, 19, 122, 133, 147
TESTRAN debugging facility, bl ,
time, calendar, 14
Time-Sharing System, PDP-10,
Time-Sharing System/360, 136,
toolsmith, 34, 128
top-down design, 134, 143
Tower of Babel, 72
TRAC language, 44
trade-off, size-function, 101
size-speed, 99, 101
training, time for, 18
Trapnell, F. M., vii, viii
tree organization, 79
Truman, H. S., 61
Tukey, J. W., 102
turnover, personnel, 179
USSR Academy of Sciences, viii
utility programs, 34, 128, 134
vehicle machine, 131
von Neumann, J., 170
Vyssotsky, V. A., viii, 142, 158,
Walk, K., 181
Walter, A. B., 188
Watson, T. J., Jr., v
Watson, T. J., Sr., 164
Weinberg, G. M., 188
Weinwurm, G. F., 88, 181, 182,
Wells Apocalypse, 61
Wilson, T. A., 187
Wirth, N., 143, 186
Witterrongel, D. M., 187
Wolverton, R. W., 179, 183
The Mythical Man-Month:
Essays on Software Engineering
by Frederick P. Brooks, Jr., University of North Carolina at Chapel Hill
About the Book
An eminent computer expert, Brooks has written a collection of thought-provoking essays on the
management of computer programming projects. These essays draw from his own experience
as project manager for the IBM System/360 and for OS/360, its operating system.
In the essays, the author blends facts on software engineering with his own personal opinions
and the opinions of others involved in building complex computer systems. He not only gives the
reader the benefit of the lessons he has learned from the OS/360 experience, but he writes
about them in an extremely readable and entertaining way.
Although formulated as separate essays, the book expresses a central argument. Brooks
believes that large programming projects suffer management problems different in kind from
small ones due to the division of labor. For this reason he feels that the critical need is for
conceptual integrity of the product itself, and in essay form he explores both the difficulties of
achieving this unity and the methods for achieving it.
ADDISON-WESLEY PUBLISHING COMPANY
Reading, Massachusetts • Menio Park, California
London • Amsterdam • Don Mills, Ontario • Sydney